What Size Radiator Do I Need? BTU Guide for UK Homes
Published 15 November 2026 · DIY & Home
Try it nowBTU Calculator →Radiator output is measured in BTU per hour or watts. Get a figure for your room with the BTU calculator.
What affects heat needed
- Room volume – length × width × height.
- Room type – living rooms and bathrooms are kept warmer than bedrooms.
- Insulation – solid walls and single glazing need much more heat.
- Outside walls and large windows.
Quick guide (average insulation, 2.4 m ceilings)
| Room | Size | Approx BTU |
|---|---|---|
| Small bedroom | 3 × 3 m | 3,000 |
| Double bedroom | 4 × 3.5 m | 4,500 |
| Living room | 5 × 4 m | 8,000 |
| Bathroom | 2.5 × 2 m | 2,500 |
Heat pumps need bigger radiators
Heat pumps run at lower flow temperatures, so radiators often need to be 2–2.5 times larger for the same heat output.
Converting
1 watt = 3.412 BTU/h. Radiator spec sheets quote output at ΔT50 – check that figure.
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