How to work out the BTU a room needs
A room needs enough heat to replace what it loses on a cold day. That heat escapes two ways: through the walls, windows, floor and ceiling, and with the air that leaks out and is replaced by cold air from outside. This BTU calculator adds up both, then gives the answer in BTU per hour and in watts, plus the radiator size to buy.
Heat lost through surfaces
Each surface loses U-value × area × temperature difference, in watts. The U-value says how easily heat passes through: about 2.1 for an uninsulated solid brick wall, 0.5 for an insulated cavity wall, and 2.8 for older double glazing.
Heat lost with the air
Ventilation loss = 0.33 × air changes per hour × room volume × temperature difference. A living room is taken as changing its air 1.5 times an hour and a bathroom 3 times, which is why bathrooms need more heat than their size suggests.
Watts to BTU
Multiply watts by 3.412 to get BTU per hour. A 1,000W radiator gives out 3,412 BTU/hr. UK radiator catalogues list both, but manufacturers rate the output at ΔT50, which matters a lot for heat pumps.
Worked example: a 4m × 3.5m living room
Two external walls, insulated cavity, older double glazing, ground floor, heated room above. 21°C inside, −3°C outside.
| Where the heat goes | Watts | Share |
|---|---|---|
| External walls | 162 W | 15% |
| Windows | 302 W | 27% |
| Floor | 235 W | 21% |
| Ventilation | 399 W | 36% |
| Total | 1,099 W (3,749 BTU) | 100% |
That is about 268 BTU per m² of floor. The ceiling loses nothing here because there is a heated room above.
Why insulation changes the answer so much
The same living room, changing only the walls or windows.
| Construction | Heat loss | BTU per hour |
|---|---|---|
| New build walls, modern glazing | 876 W | 2,990 |
| Insulated cavity, older double glazing | 1,099 W | 3,749 |
| Insulated cavity, single glazing | 1,315 W | 4,486 |
| Unfilled cavity, older double glazing | 1,423 W | 4,854 |
| Solid brick, older double glazing | 1,617 W | 5,518 |
Filling the cavity cuts this room’s heat loss by 324W on a cold day. That is why rule-of-thumb BTU figures based only on room size are so often wrong.
Radiator sizes for boilers and heat pumps
What ΔT50 means
Radiator output depends on how much hotter the water is than the room. Catalogues quote it at ΔT50 — roughly 70°C water in a 20°C room, which suits a traditional boiler. Run cooler water and the same radiator gives out much less.
Sizing for a heat pump
At ΔT30 a radiator gives about 51% of its rated output, and at ΔT20 about 30%. So the example living room needs a 1,099W radiator with a boiler, about 2,135W of rated output at ΔT30, and about 3,616W at ΔT20. Choose Heat pump above to see the size for your room.
Compare the running costs with the heating cost calculator, and read heat pump vs boiler for why flow temperature matters.
Bedrooms, bathrooms and kitchens
Bedrooms
Bedrooms are designed to 18°C and change their air more slowly. A 3.5m × 3m bedroom with one external wall and an insulated loft above needs about 430W, or 1,469 BTU — far less than the living room.
Bathrooms
Bathrooms are kept at 22°C with fast air changes for moisture. A 2.5m × 2m bathroom with one external wall needs about 424W (1,446 BTU) — almost the same as a bedroom twice its size. Most of that is ventilation, which is why a towel rail alone often leaves a bathroom cool.
Kitchens
Kitchens are designed to 18°C because cooking and appliances add heat, but extractor fans raise the air change rate. Choose Kitchen as the room type and the calculator applies both.
Getting a reliable figure
Inputs that matter most
- Wall type — solid, unfilled cavity or insulated
- Window size and glazing age
- Whether the space above or below is heated
- The outside design temperature for where you live
When to get a survey
This is a good guide for choosing a radiator. For a whole new heating system, especially a heat pump, installers carry out a room-by-room heat loss calculation using your exact construction and location. Before that, improving insulation lowers every room’s figure — see cutting an energy bill for where it sits in the order of savings.