Build & renovate
Radiator BTU Calculator
Radiators are sized from room volume: living spaces want roughly 50 W per cubic metre, bedrooms 40, kitchens and halls 30.
Calculator
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How it works
Volume x factor gives watts (x3.412 for BTU/hr). Add ~15% for north-facing or poorly insulated rooms; subtract ~10% for well-insulated new builds. Real heat-loss calcs go deeper, but this sizes the radiator correctly for most rooms.
Frequently asked questions
How many BTU for a bedroom?
A 4 x 3 m bedroom with 2.4 m ceilings: 28.8 m3 x 40 W = ~1,150 W, about 3,900 BTU/hr - a standard 600 x 1000 double-panel radiator.
Is a bigger radiator wasteful?
No - output is controlled by the thermostat and TRV. A slightly oversized radiator runs at lower flow temperatures, which is exactly what condensing boilers and heat pumps want.