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.
Results
Fill in the details and select Calculate to see your figures.
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Get a free demoHow 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 to work it out yourself
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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Get a free demoFrequently asked questions
How many BTUs do I need for a room?
Work out the room volume (length × width × height in metres), multiply by a watts-per-cubic-metre factor for the room type (around 45 for bedrooms, 55 for living rooms, 65 for bathrooms), then convert to BTU/hr by multiplying the watts by 3.412. A typical 4 m × 3 m bedroom needs roughly 4,400 BTU/hr.
What is the difference between Watts and BTU?
Both measure heat output. Watts are the metric unit and BTU/hr is the imperial unit still widely used for radiators. They are directly convertible: 1 watt equals 3.412 BTU/hr, so a 1,000 W radiator is rated at 3,412 BTU/hr.
Why does a bathroom need more heat per cubic metre?
Bathrooms are typically kept warmer, lose heat through tiling and extraction, and people expect them to feel comfortable when wet, so the rule of thumb uses a higher watts-per-cubic-metre factor than a bedroom.
Should I round up or down when choosing a radiator?
Round up. Pick a radiator rated at or just above the calculated output so the room reaches temperature comfortably. A slightly larger radiator run at a lower flow temperature is more efficient than an undersized one working flat out.
Does this replace a heat-loss survey?
No. This is a rule-of-thumb estimate based on room volume and does not account for your walls, insulation, orientation or draughts. For a system design or heat-pump specification, a full heat-loss survey by a qualified engineer is definitive.
Can Tradehand connect me with a heating engineer?
Yes. Tradehand gives UK heating businesses a whole office team without hiring anyone, answering every enquiry, sending quotes and booking the work, so a qualified engineer can survey your rooms and specify the right radiators. Get a free demo to see how it links customers to local heating professionals.
