What size boiler do I need for a 3-bedroom house?

With one bathroom, the published tables put a 3-4 bedroom home at 28-34 kW for a combi (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes) and 18-26 kW for a system or regular boiler. A second bathroom keeps it in the same row; a third moves it to 35-42 kW. The calculator picks the row from your bedrooms and bathrooms.

How do I know what size boiler I need?

Count the bedrooms and bathrooms for the table row, then check the whole-house heat loss the tool gives from your floor area and insulation: if the heat loss in kW is well below the combi range, the range is driven by hot water, not heating, which is normal for a combi. An installer confirms it with a heat loss calculation before ordering.

How many BTU does a radiator need for a room?

Volume in cubic feet x the room factor, plus the percentages. A 4 x 4 x 2.4 m living room is 1,356 ft³ x 5 = 6,800 BTU/h before adjustments, 7,500 BTU/h with one external wall (Turnbull & Co, BTU Calculator for Radiators, rule of thumb). Radiators are sold by their BTU or watt output at a stated temperature difference; pick one at or above the figure.

Why does the tool show two radiator figures?

Because two publishers state two rules: Turnbull & Co, BTU Calculator for Radiators, rule of thumb uses a factor by room type with percentage adjustments, while BestHeating, BTU Calculator, a second rule of thumb uses volume x 4 for every room. Rather than pick one, the tool shows both; the true figure for your room comes from a heat loss calculation.

Free calculator

Boiler and radiator size calculator (UK, kW and BTU)

A 90 m² three-bedroom house with one bathroom and average insulation loses about 9.00 kW (30,700 BTU/h) and sits in the 28-34 kW combi row of the published tables; its 4 x 4 m living room wants a radiator of about 7,500 BTU/h (2,186 W). Type your own house and room to see yours.

Region and unitsSwitching region converts your dimensions; the charts are the same. Your numbers stay in this browser only.

Floor area, whole house (sq ft)sq ftStarting value: BTUtally starting example, read 2026-09-20; edit it.Insulation

Heat loss per m²: 100 W/m² for this level (TradeCalculator.co.uk, Radiator Sizing Calculator, rule of thumb for whole-building heat loss per m² of floor area).

BedroomsStarting value: Heatable, What Size Boiler Do I Need?, read 2026-09-20; edit it.BathroomsStarting value: Heatable, What Size Boiler Do I Need?, read 2026-09-20; edit it.Boiler type

One room, for its radiator

Length (ft)ftStarting value: BTUtally starting example, read 2026-09-20; edit it.Width (ft)ftHeight (ft)ftStarting value: BTUtally starting example, read 2026-09-20; edit it.

Room type

Single-glazed windows+10 % each; double glazing adds nothing (Turnbull & Co, BTU Calculator for Radiators, feature adjustments).External walls+10 % per wall.

North-facing+10 %Poorly insulated room+15 %French doors or a large glass area+15 %

Result

Combi boiler: 28-34 kW
Whole-house heat loss: 9.00 kW (30,700 BTU/h)
Heat loss in watts: 9,002 W at 100 W/m²
Radiator for the living room, dining room or office: 7,500 BTU/h (2,185 W)
Room adjustments: +10 % on 1,356 ft³ x 5
Same room, flat x 4 rule: 5,400 BTU/h (1,589 W)

Boiler range: Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes for 3 bedrooms and 1 bathroom. Radiator: Turnbull & Co, BTU Calculator for Radiators, rule of thumb; the flat rule is BestHeating, BTU Calculator, a second rule of thumb. 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.

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

How it works

Three published rules, shown side by side. Whole-house heat loss = floor area x a W/m² figure for the insulation level (130 W/m² poor, 100 W/m² average, 80 W/m² good), shown in kW and BTU/h. The boiler kW range comes from the bedrooms and bathrooms row of the installers' tables: combi 24-27 kW for 1-2 bedrooms with one bathroom, 28-34 kW for 3-4 bedrooms or two bathrooms, 35-42 kW for 5+ bedrooms or three bathrooms; system or regular boilers 12-18 kW, 18-26 kW and 27-40 kW for the same rows. The combi figure is higher because it also heats the hot water on demand.

The radiator for one room = the room's volume in cubic feet x a factor by room type (kitchen 3, bedroom 4, living room and hallway 5, bathroom 6, conservatory 6.5), plus percentages added together: 10 % north-facing, 10 % per single-glazed window, 10 % per external wall, 15 % poor insulation, 15 % French doors. Watts = BTU/h / 3.412. A second published rule uses volume x 4 for every room; the result shows both because the two publishers disagree, and the range between them is the honest answer.

With the starting house, a second bathroom moves the combi range to 28-34 kW; the same house with a hot water cylinder needs a 18-26 kW system boiler. Make the living room north-facing with two single-glazed windows and its radiator goes from 7,500 BTU/h to 9,500 BTU/h (+40 % in all).

These are rules of thumb from installers and merchants, labelled as such in the sources; a room-by-room heat loss calculation by a Gas Safe engineer replaces them before a boiler is ordered. Every number is yours to change; all calculations run in your browser and the inputs stay there, so the Pro report can pick them up. 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.

Sources of the starting values

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
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

The US setting shows the same rules in sq ft and ft; the W/m² and the room factors are applied after conversion.

Frequently asked questions

What size boiler do I need for a 3-bedroom house?: With one bathroom, the published tables put a 3-4 bedroom home at 28-34 kW for a combi (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes) and 18-26 kW for a system or regular boiler. A second bathroom keeps it in the same row; a third moves it to 35-42 kW. The calculator picks the row from your bedrooms and bathrooms.
How do I know what size boiler I need?: Count the bedrooms and bathrooms for the table row, then check the whole-house heat loss the tool gives from your floor area and insulation: if the heat loss in kW is well below the combi range, the range is driven by hot water, not heating, which is normal for a combi. An installer confirms it with a heat loss calculation before ordering.
How many BTU does a radiator need for a room?: Volume in cubic feet x the room factor, plus the percentages. A 4 x 4 x 2.4 m living room is 1,356 ft³ x 5 = 6,800 BTU/h before adjustments, 7,500 BTU/h with one external wall (Turnbull & Co, BTU Calculator for Radiators, rule of thumb). Radiators are sold by their BTU or watt output at a stated temperature difference; pick one at or above the figure.
Why does the tool show two radiator figures?: Because two publishers state two rules: Turnbull & Co, BTU Calculator for Radiators, rule of thumb uses a factor by room type with percentage adjustments, while BestHeating, BTU Calculator, a second rule of thumb uses volume x 4 for every room. Rather than pick one, the tool shows both; the true figure for your room comes from a heat loss calculation.
What is W/m² and which figure applies to my house?: Watts of heat lost per square metre of floor on a cold day. The rule of thumb is 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 one (TradeCalculator.co.uk, Radiator Sizing Calculator, rule of thumb for whole-building heat loss per m² of floor area). Pick the level that matches your walls, loft and glazing; a solid-wall Victorian house with single glazing is the poor level.