The UK radiator sizing method
UK radiator sizing works from room volume: length × width × height in metres, multiplied by roughly 40–45 watts per cubic metre and corrected for the room's target temperature (living rooms are kept warmer than bedrooms), window losses and wall insulation. The result converts to BTU at 1 W = 3.412 BTU — the unit most UK radiator listings still use.
Check the ΔT50 rating when you buy
Radiator brochures quote output at ΔT50 — water 50°C hotter than the room. Modern condensing boilers running low flow temperatures (and especially heat pumps at 45°C flow) deliver less than the ΔT50 figure: at ΔT30 a radiator gives only about 63% of its rated output. If you plan a heat pump or run your boiler at 50–55°C flow for efficiency, size radiators 30–50% above this calculator's number or choose taller/double-panel models.
| Room example | Typical need | Radiator suggestion |
|---|---|---|
| Double bedroom 4×3.5 m | ~4,500–5,500 BTU | 1 × K2 600×1000 mm |
| Living room 5×4 m | ~7,500–9,000 BTU | 2 × K2 600×800 mm |
| Bathroom 2.5×2 m | ~2,500–3,000 BTU | Towel rail 1200×500 + spur |
| Kitchen 4×3 m | ~3,500–4,200 BTU | 1 × K1/K2 600×900 mm |
Frequently asked questions
How many BTU do I need per room?
Multiply room volume (m³) by ~42 W and by factors for room type, windows and insulation, then convert to BTU (×3.412). A typical double bedroom lands around 4,500–5,500 BTU; living rooms 7,000–10,000 BTU.
What does ΔT50 mean on radiators?
Output measured with the radiator water 50°C above room temperature. At lower flow temperatures (condensing boilers, heat pumps) the real output drops — about 63% at ΔT30 — so size up accordingly.
Is it OK to oversize a radiator?
Slightly oversized radiators are actually good practice: they let you run lower water temperatures, which makes condensing boilers and heat pumps more efficient. Avoid extreme oversizing in small rooms with TRVs, which then cycle.
Should I fit one big radiator or two smaller ones?
Above roughly 6,000 BTU per room, two radiators on different walls give more even warmth and fit under windows better than one very large panel.