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Radiator BTU Requirement Calculator

Size your radiator properly so the room actually reaches temperature.

How it is calculated

The tool works from the room's three dimensions in metres and a heating factor in watts per cubic metre. Volume is V = length × width × height (m³). Required output is P = V × factor in watts, and BTU per hour is P × 3.412142. The factor is whatever you enter: the form suggests roughly 40 for a lounge, 30 for a bedroom, 35 for a kitchen and 50 for a bathroom. Only a blank factor falls back to 40; a factor you type that is zero, negative or above 500 shows no result. Length and width must be above zero and no more than 100 m, and height above zero and no more than 20 m. The factor is a rule-of-thumb figure, so a room with large windows, poor insulation or several outside walls may need a higher one. Results are rounded to whole watts and BTU/h.

Worked example

A bedroom 4 m long, 3.5 m wide and 2.4 m high, with a factor of 30 W/m³. Volume: 4 × 3.5 × 2.4 = 33.6 m³. Power: 33.6 × 30 = 1,008 W. Convert: 1,008 × 3.412142 = 3,439.4 BTU/h. Result: 3,439 BTU/h, which is 1,008 W, for a room of 33.60 m³. A radiator rated at least that much at your flow temperature would cover the estimate.

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Common mistakes

  • Entering dimensions in centimetres or feet. The fields are metres, so 400 for a 4 m wall is refused as an impossible room and no result appears.
  • Leaving the factor blank and assuming the room type is guessed. A blank factor becomes 40 W/m³, the lounge value, even for a bathroom; for a bathroom type 50.
  • Typing 0 as the factor to mean none. A zero factor is rejected and is not replaced by the default, so leave the field blank to get 40.
  • Comparing the watts figure with a radiator's BTU rating, or the other way round. 1 W is about 3.41 BTU/h, so use the unit that matches the catalogue.
  • Using floor area instead of volume. A room with a high ceiling needs more heat than the same floor area with a standard one, and height is part of the formula.

About this calculator

Radiator sizing starts with a room's heat loss, which depends on its volume and how much heat it loses relative to that volume — a factor that varies with window area, exposure and typical temperature demands. The estimate is required watts = room volume × heating factor, converted to BTU, where the factor is in watts per cubic metre: living rooms and bedrooms usually need a lower factor than kitchens, bathrooms or conservatories, which lose heat faster through glazing, extraction and exterior walls.

Homeowners and heating engineers size radiators this way before buying because an undersized radiator never brings the room to the target temperature no matter how long it runs, while an oversized one wastes energy overheating the space and cycles the boiler inefficiently — getting the BTU output within the right range matters more than most people assume when shopping by radiator length alone.

If the heating factor is left blank, a default of 40 W/m³ applies, and the room volume appears next to the BTU and watt figures.

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