Radiator BTU calculator
A double bedroom of 4 × 3.5 m with a 2.4 m ceiling needs about 4,614 BTU (1,352 W) of radiator output in a typical double-glazed home.
- In watts
- 2,208 W
- Room volume
- 48.0 m³
5 × 4 m room, 2.4 m high, including a 15% margin.
Includes a 15% margin. Radiator outputs are usually quoted at ΔT50, which suits most gas boilers. For a heat pump, you will need bigger radiators: ask your installer.
BTU for common rooms
Double glazed, cavity walls, two outside walls, 2.4 m ceilings.
| Room | Size | BTU | Watts |
|---|---|---|---|
| Small bedroom | 3 × 2.5 m | 2,472 | 725 |
| Double bedroom | 4 × 3.5 m | 4,614 | 1,352 |
| Living room | 5 × 4 m | 7,534 | 2,208 |
| Kitchen | 4 × 3 m | 3,390 | 994 |
| Bathroom | 2.5 × 2 m | 2,119 | 621 |
What changes the heat a room needs
Each choice raises or lowers the starting figure for the room.
| Factor | Option | Change |
|---|---|---|
| Windows | Single glazed | +40% |
| Windows | Triple glazed | -15% |
| Type of home | Modern new build | -30% |
| Type of home | Older, poorly insulated | +20% |
| Type of home | Victorian or pre-1920s | +50% |
| Outside walls | One | -15% |
| Outside walls | Three | +15% |
| Outside walls | Four | +30% |
| Facing | North | +15% |
| Facing | South | -10% |
How we calculate this
We follow the method published by QS Supplies. The room’s volume is multiplied by a heat rate for its type: 45 W per m³ for bathrooms, 40 for living rooms, 35 for bedrooms and 30 for kitchens, where cooking adds heat.
We then adjust for windows, insulation, outside walls and which way the room faces, add a 15% margin and convert watts to BTU. For north-facing rooms, QS Supplies suggests 10 to 15% more; we use 15% so the radiator isn’t too small.
This is a quick estimate for choosing a radiator. For a new heating system or a heat pump, an installer should do a full heat loss calculation.
Questions people ask
How many BTU do I need per square metre?
It depends on the room and ceiling height, but for a typical double-glazed living room with a 2.4 m ceiling it is roughly 380 BTU per square metre. Bathrooms need more, kitchens less.
What is the difference between BTU and watts?
Both measure heat output. One watt is about 3.412 BTU per hour, so a 1,000 W radiator gives about 3,400 BTU.
Can I use two radiators instead of one big one?
Yes. Add their outputs together. Two smaller radiators on different walls can heat a large or L-shaped room more evenly.
Do I need bigger radiators with a heat pump?
Usually. Heat pumps run the water cooler than a gas boiler, so a radiator gives out less heat. Radiator outputs quoted at ΔT50 assume boiler temperatures. Your installer will size them for you.
Related calculators
- Immersion heater cost calculatorWhat it costs to heat a tank of hot water with an immersion heater.
- Heat pump running cost calculatorWhat a heat pump would cost to run in your home, compared with your gas boiler.
- Electric heater running cost calculatorWhat an electric heater costs per hour, week and winter, and how it compares with gas.
- Energy bill calculatorYour yearly and monthly gas and electricity bill at the current price cap.
- Meter reading cost calculatorWhat you’ve used and spent between two meter readings, for electricity or gas.
- Appliance running cost calculatorCost per hour, month and year for 28 common appliances, or any appliance you enter.
Save or share this calculator
Embed this calculator
Free to use on your website, blog or newsletter. Copy this code and paste it into your page.