Radiator size calculator.
Watts and BTU for a room, first pass. For heating engineers, plumbers, anyone replacing a radiator. Free, no sign-up.
About 4,586 BTU/h, for a room of 33.6 m³.
Rule-of-thumb sizing from room volume. Catalogue outputs are at ΔT50; at low flow temperatures a radiator gives less, so choose the next size up. Not a BS EN 12831 heat-loss calculation.
A first-pass figure from room volume: 30 watts per cubic metre for a well-insulated modern room, 40 for average, 50 for solid walls and single glazing. North-facing rooms get 15% more, rooms with large glazing 10% more. Radiator catalogues quote output in watts at Delta T 50; a condensing boiler running low flow temperatures delivers less, so size up rather than down.
Where it stops: this does not replace a room-by-room heat loss calculation to BS EN 12831, which is what a heat pump design or a Part L compliance check needs. Use it to sanity-check a radiator swap, not to specify a system.
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