Free Radiator BTU Calculator
Estimate the heat output a room needs in Watts and BTU/hr. Enter room dimensions, pick the room type and glazing to size your radiator.
This radiator BTU calculator estimates the heat output a room needs so you can pick a correctly sized radiator. It uses the retailer rule of thumb of watts per cubic metre (multiplying the room volume by a room-type factor, then adjusting for glazing and insulation) and converts to BTU/hr (1 watt = 3.412 BTU/hr). It's a quick guide to specify a radiator, not a replacement for a full heat-loss survey.
Radiator BTU Calculator results
Fill in the details and select Calculate to see your figures.
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Get startedHow to use the Radiator BTU Calculator
- 1Measure the room's length, width and ceiling height in metres.
- 2Select the room type, since bedrooms need less heat per cubic metre than living rooms or bathrooms.
- 3Pick the glazing option that best matches the room to adjust for heat loss through windows.
- 4Read the heat output in Watts and BTU/hr and choose a radiator rated at or just above that figure.
- 5For a room with unusual heat loss (e.g. a conservatory), get an installer to run a proper heat-loss calculation.
How the Radiator BTU Calculator works
Radiator size follows a rule of thumb based on room volume. Multiply length by width by height to get cubic metres, multiply by a watts-per-cubic-metre factor for the room type, then scale for glazing.
- A 4 m × 3 m × 2.4 m bedroom = 28.8 m³ volume.
- At the bedroom factor of 45 W/m³ with standard glazing (100%): 28.8 × 45 × 1.00 = 1,296 W.
- In BTU/hr: 1,296 × 3.412 = 4,422 BTU/hr, so pick a radiator rated at or just above this.
This is a retailer rule-of-thumb estimate. The watts-per-cubic-metre factors vary between sources, and it does not account for the specific fabric, orientation or exposure of your room. It is a guide for specifying a radiator, not a substitute for a full heat-loss survey by a qualified heating engineer, which is definitive.
Typical UK benchmarks
| Bedroom factor | ≈ 45 W/m³ rule of thumb (BestHeating BTU guide) |
| Living / dining / kitchen factor | ≈ 55 W/m³ rule of thumb (BestHeating BTU guide) |
| Bathroom factor | ≈ 65 W/m³ rule of thumb (BestHeating BTU guide) |
| Watt to BTU conversion | 1 W = 3.412 BTU/hr (standard conversion) |
Disclaimer
This tool gives an estimate for general guidance only, based on typical UK figures and the details you enter. It is not a formal quote, survey, or professional advice. Always confirm with a qualified tradesperson before relying on any figure, as regional prices, site conditions, and current standards vary.
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