What size boiler do I need for 8 or 9 radiators?

With one bathroom, a 24-27 kW combi, the first row of the installers' table (Heatable, What Size Boiler Do I Need?, radiator column of both tables); Ideal Heating's guide gives a 24 kW combi for up to 10 radiators. A system or regular boiler is 12-18 kW. A second bathroom moves the combi to 28-34 kW, a third to 35-42 kW.

What size boiler do I need for 10 radiators?

10 is where both sources move up a size. The installers' table lists 10 in its first row and its second, and the calculator takes the larger boiler: 28-34 kW for a combi or 18-26 kW for a system boiler, with one or two bathrooms. Ideal Heating's guide moves from 24 kW to 28 kW at 10 radiators.

How many kW boiler for 12 radiators?

28-34 kW for a combi with one or two bathrooms, which heats 11.5-13.9 L/min of hot water at a 35 °C rise; 18-26 kW for a system boiler. Ideal Heating's guide gives 28 kW for 10 to 15 radiators. With three bathrooms the row is 35-42 kW combi or 27-40 kW system.

How many kW boiler for 14 radiators?

The same row as 12: 28-34 kW for a combi or 18-26 kW for a system boiler with one or two bathrooms, because the installers' next row starts at 20 radiators. 14 is the last count of Ideal Heating's 28 kW band; its 32 kW band starts at 15.

What size combi boiler for 7 or 8 radiators?

With one bathroom, 24-27 kW for both, which heats 9.8-11.1 L/min at a 35 °C rise; two showers at once would share it, which is why the calculator then points to a system boiler. With a second bathroom the combi row is 28-34 kW (11.5-13.9 L/min), because a combi is sized for its hot water. A combi only heats what your mains brings in, so measure your mains flow with the bucket test on the calculator page before you pay for a bigger one.

Does the size of each radiator matter?

For the heating, yes; for this table, no. A count is a rule of thumb: 10 small bedroom radiators and 10 large living room radiators count the same. The heating side is the house's heat loss, floor area x 80 W/m² to 130 W/m² by insulation (TradeCalculator.co.uk, Radiator Sizing Calculator, rule of thumb for whole-building heat loss per m² of floor area), in the calculator's More precise; the radiator size guide sizes each radiator by room.

What size boiler do I need for 8 to 14 radiators? (UK table)

With one bathroom, 8 or 9 radiators sit in the 24-27 kW combi row of the UK installers' table and 10 to 14 radiators in the 28-34 kW row; as a system boiler that is 12-18 kW or 18-26 kW. A second bathroom puts every count from 8 to 14 in the 28-34 kW row, a third in 35-42 kW. The table gives each count with the hot water each combi row heats.

Table 1: boiler size for 8 to 14 radiators by bathrooms, combi with its hot water flow at a 35 °C rise, and system
Radiators	1 bathroom	2 bathrooms	3 bathrooms	Manufacturer's guide
8	24-27 kW combi, 9.8-11.1 L/min12-18 kW system	28-34 kW combi, 11.5-13.9 L/min18-26 kW system	35-42 kW combi, 14.3-17.2 L/min27-40 kW system	24 kW combi
9	24-27 kW combi, 9.8-11.1 L/min12-18 kW system	28-34 kW combi, 11.5-13.9 L/min18-26 kW system	35-42 kW combi, 14.3-17.2 L/min27-40 kW system	24 kW combi
10	28-34 kW combi, 11.5-13.9 L/min18-26 kW system	28-34 kW combi, 11.5-13.9 L/min18-26 kW system	35-42 kW combi, 14.3-17.2 L/min27-40 kW system	28 kW combi
11	28-34 kW combi, 11.5-13.9 L/min18-26 kW system	28-34 kW combi, 11.5-13.9 L/min18-26 kW system	35-42 kW combi, 14.3-17.2 L/min27-40 kW system	28 kW combi
12	28-34 kW combi, 11.5-13.9 L/min18-26 kW system	28-34 kW combi, 11.5-13.9 L/min18-26 kW system	35-42 kW combi, 14.3-17.2 L/min27-40 kW system	28 kW combi
13	28-34 kW combi, 11.5-13.9 L/min18-26 kW system	28-34 kW combi, 11.5-13.9 L/min18-26 kW system	35-42 kW combi, 14.3-17.2 L/min27-40 kW system	28 kW combi
14	28-34 kW combi, 11.5-13.9 L/min18-26 kW system	28-34 kW combi, 11.5-13.9 L/min18-26 kW system	35-42 kW combi, 14.3-17.2 L/min27-40 kW system	28 kW combi

Open the boiler size calculator

How the table is worked out

The UK installers' table has three rows, and the radiator count is one way it picks a row: up to 10 radiators, then 10 or more, then 20 or more (Heatable, What Size Boiler Do I Need?, radiator column of both tables). A count of exactly 10 or 20 sits in two rows of the source, and the calculator takes the larger boiler. Bathrooms pick a row too: one bathroom the first, two the second, three or more the third (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes).

The radiator count is one of the readings the calculator combines. It takes the larger of the radiator row and the bathroom row, because a combi is sized for its hot water as well as the heating. The bedrooms give another row, and the floor area gives the heat loss; the boiler size calculator shows them side by side, so you can see when they disagree.

The last column is a second rule of thumb, Ideal Heating's guide, which gives one combi output per band of radiators (Ideal Heating, Combi Boiler Size Guide, radiators a combi output serves): 24 kW for up to 10, 28 kW for 10 to 15, 32 kW for 15 to 20. For 8 to 14 radiators that is 24 kW for 8 or 9 and 28 kW for 10 to 14.

The hot water figure is what a combi at each end of the row heats a minute through a 35 °C rise: kW x 60 / (4.184 x 35) litres, about kW / 2.44 (specific heat of water from Wikipedia; manufacturers quote combi flow at that rise). The three combi rows heat 24-27 kW 9.8-11.1 L/min, 28-34 kW 11.5-13.9 L/min, 35-42 kW 14.3-17.2 L/min.

Combi or system boiler with 8 to 14 radiators

The radiators do not decide the type; the hot water does. One bathroom, or two used one at a time, suits a combi. Two showers running at once, or three bathrooms, point to a system boiler with a cylinder: with three bathrooms the source adds "often consider a system boiler" to the 35-42 kW combi row (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes). The rules the calculator applies, and what each type needs, are on the system boiler vs combi boiler page.

A count is not a heat loss calculation

Both readings count radiators, not their size. The heating side is the house's heat loss, 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. To size each radiator in watts and BTU by room, use the radiator size guide; for the whole house, type your floor area in the calculator's More precise section.

The other way round: how many radiators a boiler runs

If you already have a boiler and want to know how many radiators it can run, the question is turned round: a 28 kW combi is the manufacturer's output for 10 to 15 radiators. Every output side by side is on how many radiators a 28 kW boiler can run, and the calculator page has the radiators for every combi output from 24 to 40 kW.

Frequently asked questions

What size boiler do I need for 8 or 9 radiators?: With one bathroom, a 24-27 kW combi, the first row of the installers' table (Heatable, What Size Boiler Do I Need?, radiator column of both tables); Ideal Heating's guide gives a 24 kW combi for up to 10 radiators. A system or regular boiler is 12-18 kW. A second bathroom moves the combi to 28-34 kW, a third to 35-42 kW.
What size boiler do I need for 10 radiators?: 10 is where both sources move up a size. The installers' table lists 10 in its first row and its second, and the calculator takes the larger boiler: 28-34 kW for a combi or 18-26 kW for a system boiler, with one or two bathrooms. Ideal Heating's guide moves from 24 kW to 28 kW at 10 radiators.
How many kW boiler for 12 radiators?: 28-34 kW for a combi with one or two bathrooms, which heats 11.5-13.9 L/min of hot water at a 35 °C rise; 18-26 kW for a system boiler. Ideal Heating's guide gives 28 kW for 10 to 15 radiators. With three bathrooms the row is 35-42 kW combi or 27-40 kW system.
How many kW boiler for 14 radiators?: The same row as 12: 28-34 kW for a combi or 18-26 kW for a system boiler with one or two bathrooms, because the installers' next row starts at 20 radiators. 14 is the last count of Ideal Heating's 28 kW band; its 32 kW band starts at 15.
What size combi boiler for 7 or 8 radiators?: With one bathroom, 24-27 kW for both, which heats 9.8-11.1 L/min at a 35 °C rise; two showers at once would share it, which is why the calculator then points to a system boiler. With a second bathroom the combi row is 28-34 kW (11.5-13.9 L/min), because a combi is sized for its hot water. A combi only heats what your mains brings in, so measure your mains flow with the bucket test on the calculator page before you pay for a bigger one.
Does the size of each radiator matter?: For the heating, yes; for this table, no. A count is a rule of thumb: 10 small bedroom radiators and 10 large living room radiators count the same. The heating side is the house's heat loss, floor area x 80 W/m² to 130 W/m² by insulation (TradeCalculator.co.uk, Radiator Sizing Calculator, rule of thumb for whole-building heat loss per m² of floor area), in the calculator's More precise; the radiator size guide sizes each radiator by room.

Sources

Heatable, What Size Boiler Do I Need? (Boiler kW Calculator) (undated), read 2026-09-20.
Ideal Heating, Combi Boiler Size Guide (undated), read 2026-10-12.
Worcester Bosch, Technical Bulletin TB 116 B: NHBC policy for combination boilers (03/2019), read 2026-10-12.
Baxi, Baxi 800 Combi 2, technical data (undated), read 2026-10-12.
TradeCalculator.co.uk, Radiator Sizing Calculator (W/m² rule of thumb) (updated September 2026), read 2026-09-20.
Wikipedia, Specific heat capacity (undated), read 2026-10-12.
Wikipedia, Properties of water (undated), read 2026-10-12.

Every kW and L/min on this page is computed by the boiler size calculator's own code from the tables above; only the list of radiator counts is ours, from the searches people make. It is a rule of thumb, not a heat loss survey.

Every figure with its quote and read date

Boiler rows 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) groups the same way: "0-10 radiators" -> 24-27kW, "10+ radiators" -> 28-34kW, "20+ radiators" -> 35-42kW. Rule of thumb from the manufacturer's guide, 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 of both tables: "0-10 radiators" in the first row (combi "24-27kW", system "12-18kW"), "10+ radiators" in the second (combi "28-34kW", system "18-26kW"), "20+ radiators" in the third (combi "35-42kW", system "27-40kW"). A count of exactly 10 or 20 sits in two rows; 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

Hot water flow

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

What size boiler do I need? UK combi boiler size calculator
Combi boiler size calculator (UK): 24-27 kW for 1-2 bedrooms, 28-34 kW for 3-4 bedrooms or 2 bathrooms, 35-42 kW for 5+ bedrooms. Hot water L/min too.
What size radiator do I need? (BTU and watts by room size and type, UK)
BTU/h and watts for UK room sizes by type from the merchants' ft³ x factor rule, each adjustment, and the flat x4 rule. A 4 x 4 m living room: 7,500 BTU/h.
What size boiler for a 3-bedroom house? (UK: 28-34 kW combi, 18-26 kW system)
A 3-bedroom house with one or two bathrooms needs 28-34 kW from a combi or 18-26 kW from a system boiler on the UK tables; heat loss by floor area.
What size boiler for a 4-bedroom house? (UK: 28-34 kW combi, 35-42 kW with 3 bathrooms)
A 4-bedroom house is 28-34 kW combi or 18-26 kW system with one or two bathrooms, 35-42 kW combi with three. Combi or system, heat loss, radiators.

Last updated 2026-10-19. Sizing estimates from published charts and rules of thumb (ENERGY STAR, manufacturer guides), each with its source and date: a Manual J load calculation by an installer is the final word.