Trades & DIY
Radiator BTU & Wattage Calculator
Find how much heat output a room needs so you can pick the right size radiator.
- Room volume
- 37.80 m³
- Heat required
- 6,362 BTU/hr
- Heat required
- 1,865 W
- Per radiator
- 1,865 W / 6,362 BTU
How the BTU calculation works
We start with the room's volume: length × width × height. A typical UK room with average insulation needs roughly 153 BTU/hr per m³ to hold its target temperature on a cold winter day.
That base figure is then adjusted: rooms kept warmer (bathrooms) need more, bedrooms and halls less. Poor insulation adds around 25%, single glazing about 15%, and each external wall around 10%.
To convert, watts = BTU ÷ 3.412. Radiators in the UK are rated at ΔT50; modern low-temperature systems and heat pumps run cooler (ΔT30–40), so the radiator must be considerably bigger — roughly 1.5–2× the ΔT50 output.
This is an estimate. For a heat pump or full system design, a room-by-room heat loss calculation (e.g. to MCS / BS EN 12831) is recommended.
Typical heat requirement by room size (average insulation, 2.4m ceiling)
| Room size | Living room (W) | Bedroom (W) | Bathroom (W) |
|---|---|---|---|
| 3 × 3 m | 969 | 823 | 1,065 |
| 3 × 4 m | 1,291 | 1,098 | 1,421 |
| 4 × 4 m | 1,722 | 1,464 | 1,894 |
| 4 × 5 m | 2,152 | 1,830 | 2,368 |
| 5 × 6 m | 3,229 | 2,744 | 3,551 |