Radiator BTU calculator

How this is calculated

This is a rule-of-thumb estimate. Published guides use roughly 30 to 50 watts per cubic metre depending on the room, insulation and glazing, so results differ between calculators. The calculator starts from the room volume (length x width x height), multiplies by a base figure for the room type — living room 40, bedroom 35, bathroom 45 and kitchen 30 watts per m³ — then adjusts for glazing, insulation and the number of external walls, and adds your safety margin. Living rooms are usually heated warmer than bedrooms, which is why the base figures differ.

Radiators are sold with outputs in watts or BTU per hour. The conversion is exact: BTU/h = watts x 3.412. The calculator shows watts, BTU/h and kW so you can match whichever figure the radiator listing uses.

Add a margin and round up to the next available radiator size; a slightly larger radiator can always be turned down. For a precise figure, use a full room-by-room heat loss calculation by a heating engineer. Hallways and conservatories are not covered by the rates here.

• Rule-of-thumb estimate; published guides use roughly 30 to 50 watts per m³, so results differ between calculators.
• Hallways and conservatories are not covered by the rates here.