BTUtally

What size boiler do I need? UK combi boiler size calculator

A combi boiler for a UK home is 24-27 kW for 1-2 bedrooms and 1 bathroom, 28-34 kW for 3 bedrooms (4 with 1 bathroom) or 2 bathrooms, and 35-42 kW for 5+ bedrooms (4 with 2+ bathrooms) or 3+ bathrooms on the installers' tables. Type your home below for your row, the boiler type and the hot water flow.

Your home
Count en-suites and shower rooms as bathrooms. Radiators are optional: they give a second reading.
Showers or baths running at the same time
A bath filling while someone showers counts as two.

Combi boilerheats water from the mains as you use it, no cylinder

28-34kW

One bathroom: a combi heats water as you use it, the type Worcester Bosch calls "perfect for smaller homes with one bathroom".

Hot water
11.5-13.9 L/min
at a 35 °C rise
Heating need
9.0 kW
90 m², average insulation
  • Your combi row: 28-34 kW
  • Other rows: 24-27 kW, 35-42 kW
  • Heating need: 9.0 kW
  • Gap to your row: output for hot water

What this means

  • A combi's kW is its hot water output: 28-34 kW warms 11.5-13.9 L/min by 35 °C, the rise Baxi's datasheet and Worcester Bosch's TB 116 B quote combi flow at.
  • For new homes the NHBC Standards 2019 ask 12.0 L/min for a single bathroom with a shower over the bath; Worcester Bosch pairs that with at least 30 kW to hot water, inside this row.
  • Heating needs about 9.0 kW (90 m², average insulation), well under the row: the hot water sets a combi's size.
  • Bigger is not better: in a study of 221 UK combis most appeared oversized for space heating, and half of them averaged more than 50 starts a day (Bennett, Elwell and Oreszczyn, 2018).

Other readings

By 10 radiators and 1 bathroom (manufacturer's guide: 28 kW)
28-34 kW
A system boiler with a cylinder instead
18-26 kW

Rows: Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes. Flow: kW x 60 / (4.184 x 35), within 5 % of the Baxi and Worcester Bosch datasheets.

Before you buy, a Gas Safe registered engineer confirms the size with a heat loss survey: find one on the Gas Safe Register.

Sizing the whole home? Pro puts every room on one printable report.

A room list with cooling BTU per room and the totals, the heating load, the boiler kW and the equipment classes, with the sources. Printed or exported to Excel.

See Pro

From $4.99 a month, or $19 once

Hot water check (optional)
How to measure it: the bucket test
  1. Turn off every other tap, the shower and any appliance using water.
  2. Open the kitchen cold tap fully (the first tap off the supply pipe) and time how long it takes to fill a 4.5 litre bucket.
  3. Type the time below. 4.5 litres in 30 seconds is 9.0 L/min; Affinity Water says a slower fill likely means a pressure or flow problem.
Your flow: type the time
More precise: heat loss and one radiatorFloor area, insulation and one room, in m² and m
Heat loss = floor area x 100 W/m² at this level (TradeCalculator.co.uk, Radiator Sizing Calculator, rule of thumb for whole-building heat loss per m² of floor area).
One room, for its radiator
Whole-house heat loss
9.00 kW (30,700 BTU/h)
Radiator for the living room, dining room or office
7,500 BTU/h (2,186 W)
Room adjustments
+10 % on 1,356 ft³ x 5
Same room, flat x 4 rule
5,400 BTU/h (1,590 W)
Radiator: Turnbull & Co, BTU Calculator for Radiators, rule of thumb; the flat rule is BestHeating, BTU Calculator, a second rule of thumb. The two publishers disagree, so both figures show.

Combi boiler size chart (UK)

The installers' table has three rows (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes). Bedrooms and bathrooms each pick a row and the larger one counts. The hot water column is what a combi at each end of the row heats a minute at a 35 °C rise.

Combi boiler size chart (UK): kW by bedrooms, bathrooms or radiators, with the hot water flow
HomeRadiatorsCombiHot waterSystem or regular
1-2 bedrooms and 1 bathroomup to 1024-27 kW9.8-11.1 L/min12-18 kW
3 bedrooms (4 with 1 bathroom) or 2 bathrooms10 to 2028-34 kW11.5-13.9 L/min18-26 kW
5+ bedrooms (4 with 2+ bathrooms) or 3+ bathrooms20 or more35-42 kW14.3-17.2 L/min27-40 kW

Every bedrooms and bathrooms pair is in Table 1. A second bathroom moves a small home up a row: a 2-bedroom home with two bathrooms is 28-34 kW, not 24-27 kW. Four bedrooms sit in two rows of Heatable's tables ("3-4" and "4+"): the calculator puts them in the 28-34 kW row with one bathroom and the 35-42 kW row with two or more, because Heatable's house-type table, "a rough guide", gives a "4 Bedroom House" a 30kW minimum combi with "1 Bath/Shower" and 35kW with "2 Baths/Showers". Worked pages: what size boiler for a 3-bedroom house and what size boiler for a 4-bedroom house.

Table 1: combi boiler kW by bedrooms and bathrooms, from the UK installers' tables
Bedrooms1 bathroom2 bathrooms3 bathrooms
124-27 kW28-34 kW35-42 kW
224-27 kW28-34 kW35-42 kW
328-34 kW28-34 kW35-42 kW
428-34 kW35-42 kW35-42 kW
535-42 kW35-42 kW35-42 kW

How to read your result

  • The big figure is the row of the table your bedrooms and bathrooms pick, for the type the calculator recommends. When UK installers talk about the size of a boiler they mean its output in kW, not the size of the cabinet; 1 kW = 3,412 BTU/h.
  • The type (combi, system or regular) comes from your bathrooms, the showers you run at once, your mains flow and whether there is room for a cylinder: the rules are in the next section.
  • Hot water is what a combi at each end of the row heats a minute at a 35 °C rise. With a mains flow typed in, it is capped at your mains, because a combi cannot heat water that does not arrive.
  • Heating need is the whole house's heat loss: floor area x W/m² for the insulation, in More precise. It usually sits far below a combi's kW: even a 200 m² poorly insulated house loses 26.00 kW, because a combi is sized for the hot water.
  • The scale puts your row among the three rows of the table, with the heating need and your mains limit marked, so you can see how much of a combi's output is there for hot water.
  • Other readings give the row your radiator count picks (with the manufacturer's single output for the count) and the other type's row, so you can see when the readings disagree.

What installers get wrong, and what to ask for

In a study of 221 UK combi boilers most appeared oversized for space heating (Bennett, Elwell and Oreszczyn, 2018). Part of that is built in: a combi's kW is its hot water output, and even the smallest combi row, 24-27 kW, is far above the heat the starting house loses (90 m², 9.00 kW at average insulation). Before you accept a size up, check what it would buy:

  • Not more hot water when the mains is the limit. On a 10.0 L/min mains a 24.4 kW combi already heats all of it at a 35 °C rise; a bigger one heats the same water.
  • Not always more heating. Even a 200 m² house with poor insulation loses 26.00 kW, below the 28-34 kW row but above the 24 kW bottom of the smallest row: compare the model's central heating output with your heat loss.
  • When a size up is right. A second bathroom moves the row up (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes), and for new homes the NHBC Standards 2019 ask 15.0 L/min for a bathroom and one separate shower room or en-suite, which Worcester Bosch pairs with at least 36 kW to hot water.

When a combi is not enough

Example homes run through the calculator's rules, with the reason and its source:

  • Two bathrooms, two showers at the same time: system boiler. Two showers at once share one combi's flow; the Energy Saving Trust says its "on demand" feature "works less well when you need hot water in multiple places at once", and Worcester Bosch calls system boilers "well-suited to homes that need hot water for several outlets at the same time".
  • Three bathrooms: system boiler. Worcester Bosch's bulletin on the NHBC standards for new homes says houses with more bathrooms "are likely to require a stored water solution", and Heatable's table adds "often consider a system boiler" to its three-bathroom combi row.
  • Two bathrooms, low mains flow: regular boiler. Your mains (8.0 L/min) is under the 9.0 L/min the bucket test expects, and Worcester Bosch says traditional (regular) boilers "can operate effectively even if your mains water pressure is low" and "will deliver the same level of flow however many taps and showers are running". A regular boiler's cylinder is fed from a cold water tank, usually in the loft (Energy Saving Trust), not straight from your 8.0 L/min mains.
  • No room for a cylinder, two showers at the same time: combi boiler. Without room for a cylinder, a combi is the type that fits: the Energy Saving Trust says combis "don't use a hot water cylinder", which makes them "a good option if you don't have much space". Two showers at once share its flow.

How the types compare on space, efficiency and what drives the cost: system boiler vs combi boiler.

What to ask for

  1. A room-by-room heat loss calculation. The British Standard is BS EN 12831-1:2017, which "covers methods for the calculation of the design heat load for single rooms, building entities, and buildings" (BSI); the MCS Heat Load Calculator works "in accordance with BS EN 12831-1:2017". Compare the result with the model's central heating output.
  2. The model's hot water flow from its datasheet, and the rise it is quoted at (Baxi's 836 Combi 2 uses 35 °C, Worcester Bosch's Greenstar 4000 25kW uses 40 °C), beside your bucket test (below).
  3. The extra efficiency measure (England). Approved Document L, Volume 1, paragraph 6.2: "If a gas-fired combination boiler is installed in an existing dwelling, at least one of the following energy efficiency measures, appropriate to the system, should be installed": flue gas heat recovery, weather compensation, load compensation or smart thermostat with automation and optimisation. The rule began in April 2018 as Boiler Plus. Ask which one the quote includes; the smart thermostat guide lists all four, with what the Government's 2021 review found about each.
  4. A Gas Safe registered engineer. Under the Gas Safety (Installation and Use) Regulations 1998 a gas engineering business must be on the Gas Safe Register to legally do gas work that is within the scope of the Regulations, and engineers carry an identity card you can ask to see (HSE).

Combi, system or regular boiler?

A combi heats water from the mains as you use it and needs no cylinder. A system boiler heats a hot water cylinder fed from the mains; a regular (heat-only) boiler heats a cylinder fed from a cold water tank in the loft. The calculator applies these rules in order, the first that fits your home wins:

Which type of boiler: the rules the calculator applies, with their source
Your homeTypeWhy
No room for a hot water cylinderCombiThe only type without a cylinder: combis "don't use a hot water cylinder" (Energy Saving Trust); two showers at once share its flow.
Low mains flow and more than one bathroom or shower at onceRegularWorks even if your mains water pressure is low, with the same level of flow however many taps and showers are running (Worcester Bosch).
Three or more bathroomsSystemNew homes with more bathrooms are likely to need stored hot water (Worcester Bosch, on the NHBC standards); "often consider a system boiler" (Heatable).
Two or more showers at onceSystemHot water for several outlets at the same time (Worcester Bosch).
Two bathrooms used in turnCombi, a row upA second bathroom moves the combi row up (Heatable).
One bathroomCombiCombis suit smaller homes with one bathroom (Worcester Bosch).

A system or regular boiler heats a cylinder rather than water on demand, so its rows are lower (Heatable, What Size Boiler Do I Need?, system / regular boiler sizing for typical UK homes): 12-18 kW, 18-26 kW and 27-40 kW for the same three rows; 18-26 kW against 28-34 kW for a combi in the three-bedroom, one-bathroom starting house. Three bathrooms sit in two rows of Heatable's system table ("2-3" and "3+"); the calculator takes the larger, 27-40 kW, in Table 2. The two compared on hot water, space, efficiency and the rules for unvented cylinders: system boiler vs combi boiler.

Table 2: system or regular boiler kW by bedrooms and bathrooms, from the UK installers' tables
Bedrooms1 bathroom2 bathrooms3 bathrooms
112-18 kW18-26 kW27-40 kW
212-18 kW18-26 kW27-40 kW
318-26 kW18-26 kW27-40 kW
418-26 kW27-40 kW27-40 kW
527-40 kW27-40 kW27-40 kW

Hot water flow and mains pressure

Warming water takes 4.184 kJ per kilogram for each °C (Wikipedia, specific heat capacity), and a litre of water weighs about a kilogram. So a combi's hot water output in kW heats kW x 60 / (4.184 x 35) litres a minute through a 35 °C rise, about kW / 2.44. Manufacturers usually quote combi flow at that rise.

Table 3: combi output to hot water flow at a 35 °C rise
Combi outputHot water flowInstallers' row
24 kW9.8 L/min24-27 kW
28 kW11.5 L/min28-34 kW
30 kW12.3 L/min28-34 kW
32 kW13.1 L/min28-34 kW
35 kW14.3 L/min35-42 kW
40 kW16.4 L/min35-42 kW

The formula is within 5 % of every datasheet checked, at the rise each datasheet uses:

Table 4: the formula against manufacturers' datasheets
ModelHot water outputRiseDatasheetFormula
Baxi 824 Combi 224 kW35 °C10.2 L/min9.8 L/min
Baxi 830 Combi 230 kW35 °C12.2 L/min12.3 L/min
Baxi 836 Combi 236 kW35 °C15.0 L/min14.8 L/min
Worcester Bosch Greenstar 4000 25kW25 kW40 °C9.0 L/min9.0 L/min

How much a home needs: for new homes the NHBC Standards 2019 ask 12.0 L/min for a single bathroom with a shower over the bath and 15.0 L/min for a bathroom and one separate shower room or en-suite. Worcester Bosch's bulletin pairs those with at least 30 kW and 36 kW to hot water, and says homes with more bathrooms are likely to need stored hot water.

The mains sets the ceiling

A combi heats the water the mains brings in; it cannot add flow. On a 10.0 L/min mains a 24.4 kW combi already heats all of it, so a bigger combi gives no more hot water. The calculator's hot water check does this sum for your own mains and tells you above which kW a combi stops helping.

Measure your mains flow: the bucket test

  1. Turn off every other tap, the shower and any appliance using water.
  2. Open the kitchen cold tap fully (the first tap off the supply pipe) and time how long a 4.5 litre bucket takes to fill.
  3. Litres a minute = litres / seconds x 60. 4.5 litres in 30 seconds is 9.0 L/min; Affinity Water says a slower fill likely means a pressure or flow problem, worth raising with your water company.

With a low mains and more than one bathroom, Worcester Bosch says traditional (regular) boilers "can operate effectively even if your mains water pressure is low" and "will deliver the same level of flow however many taps and showers are running"; the cylinder is fed from a cold water tank, usually in the loft (Energy Saving Trust). A system boiler's cylinder is fed from the mains, so it does not get round a low mains flow.

How many radiators can a 24, 28, 30 or 35 kW boiler run?

Two readings, both rules of thumb: the installers' table groups radiators by row (Heatable, What Size Boiler Do I Need?, radiator column of both tables), and Ideal Heating's guide gives one combi output per radiator band (24 kW for up to 10, 28 kW for 10 to 15, 32 kW for 15 to 20).

Table 5: combi output to number of radiators and hot water flow
Combi outputInstallers' rowRadiators in that rowManufacturer's guideHot water
24 kW24-27 kWup to 10up to 109.8 L/min
28 kW28-34 kW10 to 2010 to 1511.5 L/min
30 kW28-34 kW10 to 20not listed12.3 L/min
32 kW28-34 kW10 to 2015 to 2013.1 L/min
35 kW35-42 kW20 or morenot listed14.3 L/min
40 kW35-42 kW20 or morenot listed16.4 L/min

By count and bathrooms, the calculator takes the larger of the radiator row and the bathroom row, because a combi is sized for the hot water too. A count of 10 fits two rows of the source ("0-10" and "10+"), and so does any count of 20 or more ("10+" and "20+"); the calculator takes the larger boiler. A 28 kW combi has its own worked page: how many radiators a 28 kW boiler can run. Every count from 8 to 14 with the system row and the hot water: what size boiler for 8 to 14 radiators.

Table 6: combi boiler kW by number of radiators and bathrooms, with the manufacturer's single output for the count
Radiators1 bathroom2 bathrooms3 bathroomsCombi by count
624-27 kW28-34 kW35-42 kW24 kW
824-27 kW28-34 kW35-42 kW24 kW
1028-34 kW28-34 kW35-42 kW28 kW
1228-34 kW28-34 kW35-42 kW28 kW
1528-34 kW28-34 kW35-42 kW32 kW
1828-34 kW28-34 kW35-42 kW32 kW
2035-42 kW35-42 kW35-42 kWbeyond the guide
2535-42 kW35-42 kW35-42 kWbeyond the guide

Heat loss method (more precise)

The heating side of the whole house is floor area x a W/m² figure for the insulation (TradeCalculator.co.uk, Radiator Sizing Calculator, rule of thumb for whole-building heat loss per m² of floor area): poor 130 W/m², average 100 W/m², good 80 W/m². The calculator's More precise section works it out for your floor area.

Table 7: whole house heat loss in kW by floor area and insulation
Floor areapoor (130 W/m²)average (100 W/m²)good (80 W/m²)
50 m²6.50 kW5.00 kW4.00 kW
70 m²9.10 kW7.00 kW5.60 kW
90 m²11.70 kW9.00 kW7.20 kW
120 m²15.60 kW12.00 kW9.60 kW
150 m²19.50 kW15.00 kW12.00 kW
200 m²26.00 kW20.00 kW16.00 kW

A 90 m² three-bedroom house loses 7.20 kW to 11.70 kW depending on insulation, against 28-34 kW for a combi or 18-26 kW for a system boiler. Even a 200 m² house with poor insulation loses 26.00 kW, still below the 28-34 kW combi row. For one room, the calculator sizes a radiator from the room's volume in cubic feet x a factor by room type (4 for a bedroom, 5 for a living room, 6 for a bathroom; Turnbull & Co, BTU Calculator for Radiators, rule of thumb) plus percentages for external walls, single glazing and north-facing rooms, and shows the flat x 4 rule beside it (BestHeating, BTU Calculator, a second rule of thumb). Every room size and type is in the radiator size guide.

Table 8: five example homes, the combi row by bedrooms and by radiators, the system row and the heat loss at average insulation
Example homeFloor areaRadiatorsCombi by bedroomsCombi by radiatorsSystem or regularHeat loss (100 W/m²)
1 bed, 1 bathroom50 m²624-27 kW24-27 kW12-18 kW5.00 kW
2 bed, 1 bathroom70 m²824-27 kW24-27 kW12-18 kW7.00 kW
3 bed, 1 bathroom90 m²1028-34 kW28-34 kW18-26 kW9.00 kW
4 bed, 2 bathrooms120 m²1535-42 kW28-34 kW27-40 kW12.00 kW
5 bed, 3 bathrooms200 m²2535-42 kW35-42 kW27-40 kW20.00 kW

In every row the heat loss sits below the system range, and further below the combi range: the heating side is covered, and the hot water sets the combi's size. The pairing of bedrooms with a floor area and a radiator count is BTUtally's example; every kW in the row is computed from the sources. The boiler output calculator turns each output into BTU/h and the floor area it heats.

Why a bigger boiler is not better

A study of 221 UK combi boilers found most oversized for space heating: half averaged more than 50 starts a day, and 70 % of starts lasted under 10 minutes on average (Bennett, Elwell and Oreszczyn, 2018). The authors link that cycling with lower performance and call the modulation range crucial. Ideal Heating's size guide makes the same point for buyers: you need not choose the model with the highest kW. Check your row in the calculator.

More boiler guides

Frequently asked questions

What size combi boiler do I need? (combi boiler size calculator)
On the UK installers' tables (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes): 24-27 kW for 1-2 bedrooms and 1 bathroom; 28-34 kW for 3 bedrooms (4 with 1 bathroom) or 2 bathrooms; 35-42 kW for 5+ bedrooms (4 with 2+ bathrooms) or 3+ bathrooms. At a 35 °C rise those rows heat 9.8-11.1 L/min, 11.5-13.9 L/min, 14.3-17.2 L/min of hot water. The calculator at the top of this page picks your row from bedrooms and bathrooms, says whether a combi still fits when two showers run at once, and caps the flow at your mains if you measured it.
How does a whole house boiler size calculator work out the kW?
With three readings of the house. Bedrooms and bathrooms pick a row of the installers' tables; the radiator count and bathrooms pick it again (Heatable, What Size Boiler Do I Need?, radiator column of both tables); floor area x 80 W/m² to 130 W/m² by insulation gives the heat loss (TradeCalculator.co.uk, Radiator Sizing Calculator, rule of thumb for whole-building heat loss per m² of floor area). A combi's kW is set by the hot water, kW x 60 / (4.184 x 35) litres a minute; a system boiler's sits closer to the heat loss. The 90 m² starting house reads 28-34 kW for a combi and loses 9.00 kW at average insulation.
What size boiler do I need for 11 radiators?
With one bathroom, 11 radiators sit in the 28-34 kW combi row of the installers' table, and Ideal Heating's guide gives a 28 kW combi for 10 to 15 radiators (Ideal Heating, Combi Boiler Size Guide, radiators a combi output serves). A system or regular boiler is 18-26 kW; with three bathrooms the combi row moves to 35-42 kW. A count ignores how big each radiator is, so read it beside the bedrooms row and the heat loss.
Is a 25 kW combi enough?
On the installers' table, yes for a small home: 25 kW is inside the 24-27 kW combi row, for 1-2 bedrooms and 1 bathroom with up to 10 radiators (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes). It heats 10.2 L/min at a 35 °C rise, and only what your mains brings in: on the bucket test's 9.0 L/min a combi above 22.0 kW gives no more hot water. For new homes the NHBC Standards 2019 ask 12.0 L/min for a single bathroom with a shower over the bath, which Worcester Bosch pairs with at least 30 kW to hot water. A second bathroom or a third bedroom moves the home to the 28-34 kW row.
How many radiators will a 30 kW boiler run?
On the installers' table 30 kW sits in the 28-34 kW combi row, the "10+" radiators row of Heatable's table (the next row is "20+"; Heatable, What Size Boiler Do I Need?, radiator column of both tables). Heatable's own answer to this question is narrower: "For combi boilers, models with an output of 28 to 32kW are well-suited to heat homes with 10 to 15 radiators", while "30kW heat-only and system boilers are capable of efficiently heating 15 to 20 radiators". Ideal Heating's guide does not list 30 kW: it gives 28 kW for 10 to 15 radiators and 32 kW for 15 to 20 (Ideal Heating, Combi Boiler Size Guide, radiators a combi output serves). A combi's kW is set by its hot water: 30 kW heats 12.3 L/min at a 35 °C rise. Table 5, under "How many radiators can a 24, 28, 30 or 35 kW boiler run?", has 6 outputs from 24 to 40 kW: 24, 28, 30, 32, 35, 40 kW.
Is a bigger combi boiler better?
No. A combi only heats the water your mains brings in: on a 10.0 L/min mains a 24.4 kW combi already heats all of it at a 35 °C rise. Ideal Heating says you need not choose the model with the highest kW, and a study of 221 UK combis found most oversized for heating: half of them averaged more than 50 starts a day, and 70 % of starts lasted under 10 minutes (Bennett, Elwell and Oreszczyn, 2018). Pick the row your home needs and confirm it with a heat loss survey.
Does the calculator work for an oil or condensing boiler?
Yes for the sizing: the tables do not separate condensing from non-condensing boilers, and the heat loss side (floor area x W/m²) does not depend on the fuel. An oil combi reads the combi row; an oil boiler that heats a cylinder reads the system or regular row (12-18 kW, 18-26 kW, 27-40 kW). Compare the model's heating output with your heat loss and its hot water output with your bathrooms. An OFTEC-registered technician sizes an oil boiler; a Gas Safe registered engineer a gas one.
What size boiler do I need for underfloor heating?
The rules on this page do not separate underfloor heating from radiators. The heat loss side is the same whatever delivers the heat, and a combi's size still comes from the hot water, so use the bedrooms and bathrooms row and the heat loss in More precise. The radiator count does not apply to underfloor circuits; the installer's design of the circuits sets the heating side.
How do I know what size boiler I need for sure?
Use the calculator for the row, the type and the hot water, then have a Gas Safe registered engineer do a room-by-room heat loss calculation and check your hot water demand and mains flow before the boiler is ordered. Do the bucket test first (4.5 litres from the kitchen cold tap, timed): it tells you whether a bigger combi could give more hot water at all.

Sources

  1. Heatable, What Size Boiler Do I Need? (Boiler kW Calculator) (updated 3 March 2026), read 2026-10-19.
  2. Ideal Heating, Combi Boiler Size Guide (undated), read 2026-10-19.
  3. Worcester Bosch, Technical Bulletin TB 116 B: NHBC policy for combination boilers (03/2019), read 2026-10-12.
  4. Worcester Bosch, Gas boilers (undated), read 2026-10-12.
  5. Worcester Bosch, Understanding various boiler types (undated), read 2026-10-12.
  6. Worcester Bosch, Regular boilers (undated), read 2026-10-12.
  7. Worcester Bosch, Greenstar 4000 product page (undated), read 2026-10-12.
  8. Baxi, Baxi 800 Combi 2, technical data (undated), read 2026-10-12.
  9. TradeCalculator.co.uk, Radiator Sizing Calculator (W/m² rule of thumb) (updated September 2026), read 2026-09-20.
  10. Turnbull & Co, BTU Calculator for Radiators (undated), read 2026-09-20.
  11. BestHeating, BTU Calculator (undated), read 2026-09-20.
  12. Affinity Water, Water pressure: your responsibilities (bucket test) (undated), read 2026-10-12.
  13. Bennett, Elwell and Oreszczyn, Space heating operation of combination boilers in the UK (Building Services Engineering Research and Technology) (2018), read 2026-10-12.
  14. GOV.UK, Apply for the Boiler Upgrade Scheme (undated), read 2026-10-12.
  15. Wikipedia, Specific heat capacity (last edited 3 September 2026), read 2026-10-12.
  16. Wikipedia, Properties of water (undated), read 2026-10-12.
  17. Wikipedia, British thermal unit (undated), read 2026-09-20.
  18. Gas Safe Register, The official list of Gas Safe registered businesses (undated), read 2026-10-12.
  19. BSI Knowledge, BS EN 12831-1:2017 Energy performance of buildings. Method for calculation of the design heat load (31 July 2017), read 2026-10-19.
  20. MCS, Heat Load Calculator: Introduction (undated), read 2026-10-19.
  21. GOV.UK (Ministry of Housing, Communities and Local Government), Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments (for use in England) (takes effect 15 June 2022; 2023 amendments), read 2026-10-19.
  22. GOV.UK (BEIS), Boiler Plus: New standards for domestic boiler installations from April 2018, frequently asked questions (dated 03/01/2018; final answer, Q17, dated 21/06/2018; background to Approved Document L), read 2026-10-19.
  23. Health and Safety Executive, Gas Safe Register (domestic gas) (page dated 2024-11-06), read 2026-10-19.
  24. Energy Saving Trust, Boiler types explained (last updated 20 May 2026), read 2026-10-19.

This calculator gives estimates from published tables, rules of thumb and manufacturers' data. It is not a heat loss survey: have a Gas Safe registered engineer confirm the boiler size, your hot water demand and your mains flow before you buy.

Every figure with its quote and read date

Boiler rows, heat loss and radiators

  • Heat loss per m² by insulation: TradeCalculator.co.uk, Radiator Sizing Calculator (updated September 2026), rule of thumb for whole-building heat loss per m² of floor area: "80 W/m² for a new build, 100 W/m² for an average double-glazed home, and 130 W/m² for an older, poorly-insulated property" — https://tradecalculator.co.uk/plumbing/radiator-sizing-calculator/, read on 2026-09-20
  • Combi boiler kW: Heatable, What Size Boiler Do I Need? (Boiler kW Calculator) (read 2026), combi boiler sizing for typical UK homes: 1 bathroom, 0-10 radiators, 1-2 bedrooms -> "24-27kW"; 2 bathrooms, 10+ radiators, 3-4 bedrooms -> "28-34kW"; 3+ bathrooms, 20+ radiators, 4+ bedrooms -> "35-42kW (often consider a system boiler)". Ideal Heating, Combi Boiler Size Guide (read 2026): 1 bedroom "24-27kW", 2-3 bedrooms "28-34kW"; 24kW "Up to 10" radiators, 28kW "Between 10 and 15", 32kW "Between 15 and 20" — https://heatable.co.uk/boiler-advice/what-size-boiler-for-my-home and https://idealheating.com/products/boilers/ideal-combi-boilers/combi-boiler-size-guide, read on 2026-09-20
  • System boiler kW: Heatable, What Size Boiler Do I Need? (Boiler kW Calculator) (read 2026), system / regular boiler sizing for typical UK homes: 1 bathroom, 0-10 radiators, 1-2 bedrooms -> "12-18kW"; 2-3 bathrooms, 10+ radiators, 3-4 bedrooms -> "18-26kW"; 3+ bathrooms, 20+ radiators, 4+ bedrooms -> "27-40kW" — https://heatable.co.uk/boiler-advice/what-size-boiler-for-my-home, read on 2026-09-20
  • Radiators per combi kW: Ideal Heating, Combi Boiler Size Guide (read 2026), radiators a combi output serves: 24kW "Up to 10" radiators, 28kW "Between 10 and 15", 32kW "Between 15 and 20". Heatable, What Size Boiler Do I Need? (read 2026) uses wider bands, its "No. of radiators" column reading "0-10" -> 24-27kW, "10+" -> 28-34kW, "20+" -> 35-42kW (each row also set by bathrooms and bedrooms). Rule of thumb from the manufacturer's guide and an installer's table, not a heat loss calculation — https://idealheating.com/products/boilers/ideal-combi-boilers/combi-boiler-size-guide and https://heatable.co.uk/boiler-advice/what-size-boiler-for-my-home, read on 2026-09-20
  • Boiler row by radiator count: Heatable, What Size Boiler Do I Need? (Boiler kW Calculator) (read 2026), radiator column ("No. of radiators") of both tables: "0-10" in the first row (combi "24-27kW", system "12-18kW"), "10+" in the second (combi "28-34kW", system "18-26kW"), "20+" in the third (combi "35-42kW", system "27-40kW"). A count of 10 fits two rows ("0-10" and "10+"), as does any count of 20 or more ("10+" and "20+"); BTUtally takes the larger boiler. Rule of thumb, not a heat loss calculation — https://heatable.co.uk/boiler-advice/what-size-boiler-for-my-home, read on 2026-09-20
  • Radiator BTU per room: Turnbull & Co (UK plumbing merchant), BTU Calculator for Radiators (read 2026), rule of thumb: "(Length ft x width ft x height ft) x Room Base Factor + Feature Adjustments (%) = BTU Output"; factors: "Storage Rooms, Walk-In Closets, Kitchens: Factor 3", "Bedrooms: Factor 4", "Living Rooms, Dining Rooms, Home Offices: Factor 5", "Hallways, Corridors, Landings: Factor 5", "Bathrooms and En-Suites: Factor 6", "Conservatories and Sunrooms: Factor 6 to 7" (6.5 used) — https://www.turnbull.co.uk/cms/information/btu-calculator-for-radiators/, read on 2026-09-20
  • Radiator adjustments: Turnbull & Co, BTU Calculator for Radiators (read 2026), feature adjustments: "Double Glazed Windows 0%", "Single Glazed Windows 10% per window", "External Walls 10% per wall", "North-Facing Rooms 10%", "Poor Insulation 15%", "French Doors/Large Glass Areas 15-20%" (15 used); "BTU to Watts: Divide BTU by 3.412" — https://www.turnbull.co.uk/cms/information/btu-calculator-for-radiators/, read on 2026-09-20
  • Flat rule for comparison: BestHeating, BTU Calculator (read 2026), a second rule of thumb: "A room's BTU requirement is based upon the cubic volume of the space - the height, length and the width of the room multiplied by four" (feet), before the site's own heat-loss adjustments; shown beside the Turnbull figure because the two rules differ — https://www.bestheating.com/btu-calculator, read on 2026-09-20
  • BTU/h to watts: Wikipedia, British thermal unit (read 2026): "1 W is approximately 3.412142 Btu/h"; the International Table BTU is about 1,055.06 J — https://en.wikipedia.org/wiki/British_thermal_unit, read on 2026-09-20

Hot water, boiler type and the grant

  • Specific heat of water: Wikipedia, Specific heat capacity (read 2026): water's specific heat capacity is "about 4184 J⋅kg−1⋅K−1 at 20 °C", i.e. 4.184 kJ to warm 1 kg of water by 1 °C — https://en.wikipedia.org/wiki/Specific_heat_capacity, read on 2026-10-12
  • A litre of water: BTUtally assumption from Wikipedia, Properties of water (read 2026): liquid water's density is "0.99704702(83) g/mL at 25 °C", so 1 litre of tap water is taken as 1 kg (0.3 % off) — https://en.wikipedia.org/wiki/Properties_of_water, read on 2026-10-12
  • Temperature rise for combi flow: Baxi, Baxi 800 Combi 2 technical data (read 2026): "Maximum DHW flow rate (Delta T @ 35'C rise) (l/min)". Worcester Bosch, Technical Bulletin TB 116 B (03/2019): the flow rate at the outlet "is achieved with a 35°C temperature rise from the incoming mains cold water temperature" — https://baxi.co.uk/professional/products/boilers/combi-boilers/baxi-800-combi-2 and https://www.worcester-bosch.co.uk/support/literature/download/B, read on 2026-10-12
  • Combi datasheets, 35 °C rise: Baxi, Baxi 800 Combi 2 technical data (read 2026): 824 Combi 2 "Heat Output Domestic Hot Water (DHW) - kW max" 24, "Maximum DHW flow rate (Delta T @ 35'C rise) (l/min)" 10.2; 830 Combi 2: 30 kW, 12.2 l/min; 836 Combi 2: 36 kW, 15 l/min; "Minimum Operating DHW pressure required (bar)" 0.5 — https://baxi.co.uk/professional/products/boilers/combi-boilers/baxi-800-combi-2, read on 2026-10-12
  • Combi datasheet, 40 °C rise: Worcester Bosch, Greenstar 4000 product page (read 2026): Greenstar 4000 25kW, "Central heating output (kW)" 24, "Hot water flow rate 40°c ∆T (litres per minute)" 9. Worcester names its combis by their output to hot water (TB 116 B pairs "30 kW rated output to hot water" with the "Greenstar 30i" and 36 kW with the "36CDi Compact"), so the 25kW model is read as 25 kW to hot water; the flow is stated at a 40 °C rise, so the formula is checked at 40 °C — https://www.worcester-bosch.co.uk/products/boilers/directory/greenstar-i and https://www.worcester-bosch.co.uk/support/literature/download/B, read on 2026-10-12
  • Hot water demand by bathrooms: Worcester Bosch, Technical Bulletin TB 116 B (03/2019), "NHBC policy for combination boilers": a combi "should be selected with the following minimum rated output to hot water": "0.15 L/sec (9 L/min) = ≥25 kW rated output to hot water", "0.20 L/sec (12 L/min) = ≥30 kW rated output to hot water", "0.25 L/sec (15 L/min) = ≥36 kW rated output to hot water". "Example 1 – Dwelling has single bathroom with shower over the bath. Minimum NHBC requirement is 0.2 L/sec (12 L/min)"; "Example 2 – Dwelling has Bathroom and one separate Shower room/en-suite. Minimum NHBC requirement is 0.25 L/sec (15 L/min)"; "Houses with a greater number of bathrooms or shower rooms / en-suites are likely to require a stored water solution." The NHBC standard is for new homes — https://www.worcester-bosch.co.uk/support/literature/download/B, read on 2026-10-12
  • Mains flow bucket test: Affinity Water, Water pressure: your responsibilities (read 2026), a test "only for properties that have a single supply pipe": use the first tap off the supply pipe, usually the kitchen tap, with "all other taps and appliances using water" off, and "fill a 4.5 litre (1 gallon) bucket"; "It should take 30 seconds to fill the bucket with water - If it takes longer to fill then it's likely you have a water pressure or flow issue." 4.5 litres in 30 seconds is 9 litres a minute — https://metering.affinitywater.co.uk/water-pressure/your-responsibilities, read on 2026-10-12
  • Combi boilers: Worcester Bosch, Gas boilers (read 2026): "Combi boilers are perfect for smaller homes with one bathroom and provide instant hot water" — https://www.worcester-bosch.co.uk/products/boilers/gas-boilers, read on 2026-10-12
  • System boilers: Worcester Bosch, Gas boilers (read 2026): "System boilers are more suited for larger homes with more than one bathroom and require a cylinder". Worcester Bosch, Understanding various boiler types (read 2026): "System boilers are well-suited to homes that need hot water for several outlets at the same time. They take water directly from the mains, offering strong flow rates and steady performance even when more than one tap or shower is running" — https://www.worcester-bosch.co.uk/products/boilers/gas-boilers and https://www.worcester-bosch.co.uk/boiler-types, read on 2026-10-12
  • Regular boilers: Worcester Bosch, Regular boilers (read 2026): "Traditional boilers can operate effectively even if your mains water pressure is low"; they store "the heated water in bulk and allows for multiple showers, taps, and baths to run simultaneously"; "they will deliver the same level of flow however many taps and showers are running"; "installed with two tanks", the cold water tank "usually installed as high above the boiler as possible, like in your attic or loft" — https://www.worcester-bosch.co.uk/products/boilers/regular-boilers, read on 2026-10-12
  • Three or more bathrooms: Worcester Bosch, Technical Bulletin TB 116 B (03/2019): "Houses with a greater number of bathrooms or shower rooms / en-suites are likely to require a stored water solution." Heatable, What Size Boiler Do I Need? (read 2026): the 3+ bathroom row of the combi table reads "35-42kW (often consider a system boiler)" — https://www.worcester-bosch.co.uk/support/literature/download/B and https://heatable.co.uk/boiler-advice/what-size-boiler-for-my-home, read on 2026-10-12
  • Oversized combis in UK homes: Bennett, Elwell and Oreszczyn, "Space heating operation of combination boilers in the UK: The case for addressing real-world boiler performance", Building Services Engineering Research and Technology 40(1), 2018 (CREDS summary, read 2026): from 221 UK combi boilers, "Most combi-boilers appear oversized for space heating"; "Per day, half of combi boilers studied average more than 50 starts"; "70% of starts average less than 10 min during space heating operation"; "Boiler modulation range is also crucial" — https://www.creds.ac.uk/publications/space-heating-operation-of-combination-boilers-in-the-uk-the-case-for-addressing-real-world-boiler-performance/, read on 2026-10-12
  • Highest kW not needed: Ideal Heating, Combi Boiler Size Guide (read 2026): "A Combi boiler that is too big could increase the cost"; "That doesn't mean you need to choose the model with the highest kW rating" — https://idealheating.com/products/boilers/ideal-combi-boilers/combi-boiler-size-guide, read on 2026-10-12
  • Heat pump grant: GOV.UK, Apply for the Boiler Upgrade Scheme, "What you can get" (read 2026): "£7,500 towards an air source heat pump (sometimes called an air-to-water heat pump)"; "£7,500 towards a ground source heat pump"; "You cannot get a grant for a hybrid heat pump system" — https://www.gov.uk/apply-boiler-upgrade-scheme/what-you-can-get, read on 2026-10-12
  • Extra grant off the gas grid: GOV.UK, Apply for the Boiler Upgrade Scheme, "What you can get" (read 2026): "Until March 2027, you can get an additional £1,500 towards an air source heat pump or ground source heat pump if both: your property is heated by oil or LPG; you do not have a mains gas connection at the property" — https://www.gov.uk/apply-boiler-upgrade-scheme/what-you-can-get, read on 2026-10-12
  • Who can apply for the grant: GOV.UK, Apply for the Boiler Upgrade Scheme (read 2026): "This scheme is open to people in England and Wales"; you must "own the property you're applying for" and "be replacing fossil fuel heating systems - such as oil, gas, electric or LPG"; "Contact suitable MCS certified installers to get quotes for the work" — https://www.gov.uk/apply-boiler-upgrade-scheme and https://www.gov.uk/apply-boiler-upgrade-scheme/check-if-youre-eligible, read on 2026-10-12

What to ask an installer for

  • Heat load standard: BSI Knowledge, BS EN 12831-1:2017 Energy performance of buildings. Method for calculation of the design heat load - Space heating load, Module M3-3 (31 Jul 2017): the standard "covers methods for the calculation of the design heat load for single rooms, building entities, and buildings, where the design heat load is defined as the heat supply (power) needed to maintain the required internal design temperature under design external conditions" — https://knowledge.bsigroup.com/products/energy-performance-of-buildings-method-for-calculation-of-the-design-heat-load-space-heating-load-module-m3-3, read on 2026-10-19
  • MCS Heat Load Calculator: MCS, Heat Load Calculator, Introduction (undated): "The Heat Load Calculator is a system for calculating the design heat load of buildings in accordance with BS EN 12831-1:2017"; it is designed "to estimate heat loads for correctly sizing heating systems, such as heat pumps, in domestic properties" — https://heatloadcalculator.mcscertified.com/docs/getting-started/introduction, read on 2026-10-19
  • Approved Document L, paragraph 6.2: the extra measure with a gas combi: GOV.UK, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments (for use in England; takes effect 15 June 2022), paragraph 6.2 (page 40): "If a gas-fired combination boiler is installed in an existing dwelling, at least one of the following energy efficiency measures, appropriate to the system, should be installed. a. Flue gas heat recovery. b. Weather compensation. c. Load compensation. d. Smart thermostat with automation and optimisation." Approved Document L, Energy and greenhouse gas emissions, Volume 1: Dwellings, 2026 edition (takes effect on 24 March 2027), paragraph 5.9: the same words. Background, GOV.UK (BEIS), Boiler Plus: New standards for domestic boiler installations from April 2018, frequently asked questions (dated 03/01/2018; final answer, Q17, dated 21/06/2018): "When a gas combination boiler is installed, an additional energy efficiency measure will be required." — https://assets.publishing.service.gov.uk/media/662a2e3e55e1582b6ca7e592/Approved_Document_L__Conservation_of_fuel_and_power__Volume_1_Dwellings__2021_edition_incorporating_2023_amendments.pdf and https://assets.publishing.service.gov.uk/media/6a9a8da39a177a1decf97e0d/Approved_Document_L__Volume_1_Dwellings.pdf and https://assets.publishing.service.gov.uk/media/5b2cc1e2ed915d586e2d8fe9/Boiler_Plus_Factsheet_v3.pdf, read on 2026-10-19
  • Gas Safe Register: Health and Safety Executive, Gas Safe Register (page dated 2024-11-06): "Under the Gas Safety (Installation and Use) Regulations 1998 for a gas engineering business to legally undertake gas work that is within the scope of the Regulations they must be on the Gas Safe Register." "Gas Safe registered engineers carry an identity card which consumers are encouraged to ask an engineer for before they have gas work carried out in their home." The register covers "Great Britain, Northern Ireland, the Isle of Man and Guernsey" — https://www.hse.gov.uk/gas/domestic/newschemecontract.htm, read on 2026-10-19
  • Four bedrooms: the house-type table: Heatable, What Size Boiler Do I Need? (Boiler kW Calculator) (updated 3 March 2026), "What size combi boiler by house type": "You should be calculating boiler size by the number of bathrooms/showers (as above), or, use our online quoting tool. However, as a rough guide, check out the below:"; columns "1 Bath/Shower" and "2 Baths/Showers", each headed "Minimum kW Size"; rows "2 Bedroom House | 25kW | 30kW", "3 Bedroom House | 25kW | 30kW", "4 Bedroom House | 30kW | 35kW", "5 Bedroom House | 30kW | 35kW". Its combi table lists 4 bedrooms in both its "3-4" (28-34kW) and "4+" (35-42kW) rows. BTUtally puts 4 bedrooms in the 28-34 kW row with 1 bathroom and the 35-42 kW row with 2 or more (owner decision, 2026-10-19) — https://heatable.co.uk/boiler-advice/what-size-boiler-for-my-home, read on 2026-10-19

More guides

Last updated 2026-10-19. Sizing estimates from published charts and rules of thumb, each listed with its source and the day it was read: an installer's load calculation (Manual J for a US home, a room-by-room heat loss calculation in the UK) is the final word.