Radiator Size (BTU) Calculator

Calculate the BTU output you need from a radiator for any room — so you buy the right size and avoid cold rooms or inefficient oversizing.

Room length (m)

Room width (m)

Ceiling height (m)

Room type / insulation

External walls

Window type

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Fill in the details to get your calculation.

Thermostatic radiator valves (TRVs) on every radiator allow room-by-room temperature control.

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How This Is Calculated

BTU (British Thermal Units per hour) measures heat output. Required BTU = room volume × insulation factor × external wall adjustment × glazing factor. The insulation factor ranges from 50 BTU/m³ for well-insulated modern homes to 110 BTU/m³ for uninsulated Victorian properties.

Yes — sizing a radiator slightly larger than the calculated minimum (10–15%) allows you to run your boiler at a lower flow temperature (which is more efficient) while still achieving comfortable temperatures. It also future-proofs the room if you later improve insulation or the room is used differently. A very large radiator slightly oversized is often better than a compact radiator running at maximum output.