Boiler output calculator: the kW a UK house needs by floor area, in kW and BTU/h
Boiler output is the heat a boiler delivers, in kW: a 90 m² UK house with average insulation needs 9.00 kW of output to heat it (7.20 kW well insulated, 11.70 kW poorly insulated). The installers' tables put a combi for it at 28-34 kW because a combi also heats the hot water; 1 kW is 3,412 BTU/h. The table below is the output by floor area; the calculator reads your own house.
| Floor area | poor (130 W/m²) | average (100 W/m²) | good (80 W/m²) |
|---|---|---|---|
| 50 m² | 6.50 kW | 5.00 kW | 4.00 kW |
| 70 m² | 9.10 kW | 7.00 kW | 5.60 kW |
| 90 m² | 11.70 kW | 9.00 kW | 7.20 kW |
| 120 m² | 15.60 kW | 12.00 kW | 9.60 kW |
| 150 m² | 19.50 kW | 15.00 kW | 12.00 kW |
| 200 m² | 26.00 kW | 20.00 kW | 16.00 kW |
Floor area x W/m² for the insulation level, a published rule of thumb (TradeCalculator.co.uk, Radiator Sizing Calculator, rule of thumb for whole-building heat loss per m² of floor area). This is the heating output; the combi or system row for your bedrooms and bathrooms comes from the calculator below, and Table 1 turns each output into BTU/h.
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).
Result
- Combi boiler
- 28-34 kW
- Combi boiler by 10 radiators and 1 bathroom
- 28-34 kW
- Combi output for 10 radiators, manufacturer's guide
- 28 kW
- Whole-house heat loss
- 9.00 kW (30,700 BTU/h)
- Heat loss in watts
- 9,000 W at 100 W/m²
- 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 count: Heatable, What Size Boiler Do I Need?, radiator column of both tables and Ideal Heating, Combi Boiler Size Guide, radiators a combi output serves. 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.
The same tool with the method and every source: Boiler size calculator.
1. What boiler output means (kW, input and BTU/h)
A boiler has two power figures. The output is the heat that goes into the water, for the radiators or for the taps; it is the kW the installers' tables and this site's calculator use, and the figure a quote should name. The input is the fuel burned to make that heat, and is the higher of the two. A combi's data sheet gives a separate output for central heating and for hot water. Boiler kW and radiator BTU/h are the same quantity in two units: 1 W = 3.412142 BTU/h (Wikipedia, British thermal unit), so 1 kW is 3,412 BTU/h. Table 1 lists every output the tables name, in BTU/h and as the floor area whose heat loss equals it at each insulation level (TradeCalculator.co.uk, Radiator Sizing Calculator, rule of thumb for whole-building heat loss per m² of floor area).
| Output | BTU/h | poor (130 W/m²) | average (100 W/m²) | good (80 W/m²) |
|---|---|---|---|---|
| 12 kW | 40,946 BTU/h | 92 m² | 120 m² | 150 m² |
| 18 kW | 61,419 BTU/h | 138 m² | 180 m² | 225 m² |
| 24 kW | 81,891 BTU/h | 185 m² | 240 m² | 300 m² |
| 26 kW | 88,716 BTU/h | 200 m² | 260 m² | 325 m² |
| 27 kW | 92,128 BTU/h | 208 m² | 270 m² | 338 m² |
| 28 kW | 95,540 BTU/h | 215 m² | 280 m² | 350 m² |
| 32 kW | 109,189 BTU/h | 246 m² | 320 m² | 400 m² |
| 34 kW | 116,013 BTU/h | 262 m² | 340 m² | 425 m² |
| 35 kW | 119,425 BTU/h | 269 m² | 350 m² | 438 m² |
| 40 kW | 136,486 BTU/h | 308 m² | 400 m² | 500 m² |
| 42 kW | 143,310 BTU/h | 323 m² | 420 m² | 525 m² |
The floor areas are the heating side only: a combi is picked from bathrooms and radiators as well, because it heats the hot water on demand. One output in detail, with its radiator groups: how many radiators a 28 kW boiler can run.
2. The output a house needs: floor area x W/m²
The whole-house rule of thumb is floor area x 130 W/m² poor, 100 W/m² average, 80 W/m² good (TradeCalculator.co.uk, Radiator Sizing Calculator, rule of thumb for whole-building heat loss per m² of floor area); the table at the top of the page is this rule for 50 to 200 m². The calculator's starting house, 90 m² with 3 bedrooms, 1 bathroom and average insulation, loses 9.00 kW (30,700 BTU/h); the tables give it 28-34 kW as a combi or 18-26 kW as a system or regular boiler with a cylinder. The 3-bedroom boiler guide works that house by bathrooms, radiators and floor area.
3. Why the combi output is higher: bedrooms, bathrooms and radiators
A combi heats hot water on demand, so its output is set by the bathrooms it has to serve, not by the heat loss alone. The installers' tables (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes; Heatable, What Size Boiler Do I Need?, system / regular boiler sizing for typical UK homes) have three rows, read from bedrooms and bathrooms or from the radiator count, and the calculator takes the larger reading:
| Home | Radiators | Combi | System or regular |
|---|---|---|---|
| 1-2 bedrooms and 1 bathroom | up to 10 | 24-27 kW | 12-18 kW |
| 3-4 bedrooms or 2 bathrooms | 10 to 20 | 28-34 kW | 18-26 kW |
| 5+ bedrooms or 3+ bathrooms | 20 or more | 35-42 kW | 27-40 kW |
By radiators, the starting house with 10 radiators and 1 bathroom is in the 28-34 kW combi row, and the manufacturer's guide gives it a single 28 kW output (Ideal Heating, Combi Boiler Size Guide, radiators a combi output serves). Every bedrooms and bathrooms pair, every radiator count and five example homes are in the combi boiler size calculator guide.
4. Reading an output on a data sheet or a quote
A data sheet lists the central heating output and, for a combi, the hot water output; a quote usually names one kW figure, which is the output. Compare the central heating output with the heat loss row for your floor area and insulation: it should cover it. Compare the hot water output with the bathrooms row of the tables: that is what a combi is sized on. The radiators then have to match the heat loss room by room; the radiator size guide sizes each radiator in BTU/h and watts. A Gas Safe registered installer's room-by-room heat loss calculation replaces the rule of thumb before the boiler is ordered.
Frequently asked questions
- What is boiler output?
- The heat a boiler delivers to the water, in kW: the figure the installers' tables, the quotes and this calculator use. It is lower than the boiler's input, the fuel it burns, and a combi's data sheet gives one output for central heating and one for hot water. 1 kW of output is 3,412 BTU/h (Wikipedia, British thermal unit).
- How do I calculate the boiler output I need?
- Two steps. Heating: floor area x 80 W/m² (new build), 100 W/m² (average double-glazed home) or 130 W/m² (older, poorly insulated) (TradeCalculator.co.uk, Radiator Sizing Calculator, rule of thumb for whole-building heat loss per m² of floor area); 90 m² at average insulation is 9.00 kW. Boiler: the row of the installers' tables for your bedrooms, bathrooms and radiators, 24-27 kW for 1-2 bedrooms and 1 bathroom, 28-34 kW for 3-4 bedrooms or 2 bathrooms, 35-42 kW for 5+ bedrooms or 3+ bathrooms for a combi (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes). The calculator above does both and shows them side by side; a Gas Safe installer's room-by-room heat loss calculation gives the figure to buy on.
- Is boiler output the same as boiler size?
- Yes. In UK boiler sizing "size" means output in kW, not the cabinet: a "28 kW boiler" is a boiler with a 28 kW output. The combi rows 24-27 kW, 28-34 kW and 35-42 kW and the system rows 12-18 kW, 18-26 kW and 27-40 kW are all outputs (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes).
- What is 24 kW in BTU? And 28 or 30 kW?
- 24 kW is 81,891 BTU/h, 28 kW is 95,540 BTU/h and 30 kW is 102,364 BTU/h: kW x 1,000 x 3.412142 (Wikipedia, British thermal unit). Table 1 lists every output the tables name, from 12 to 42 kW, with the floor area each one would heat.
- What output boiler do I need for a 3-bedroom house?
- 28-34 kW for a combi or 18-26 kW for a system or regular boiler with one or two bathrooms (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes). The heating alone is smaller: a 90 m² house with average insulation loses 9.00 kW, 11.70 kW if poorly insulated. The 3-bedroom boiler guide linked above works the house by bathrooms, radiators and floor area.
- Does an oil or condensing boiler need a different output?
- The heat loss side, floor area x W/m², does not depend on the fuel or the boiler type. The bedrooms, bathrooms and radiator rows are the installers' tables for combi boilers and for system or regular boilers (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes; Heatable, What Size Boiler Do I Need?, system / regular boiler sizing for typical UK homes), and they do not separate condensing from non-condensing models. An oil boiler with a hot water cylinder reads the system or regular row (18-26 kW for the starting house); an OFTEC-registered technician's heat loss calculation decides. For a gas boiler it is a Gas Safe registered installer.
More guides
- What size combi boiler do I need? UK boiler kW calculator by bedrooms, radiators and bathrooms
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. By radiators 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 2026-10-03. 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.