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.
Radiator BTU Calculator results
Fill in the details and select Calculate to see your figures.
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.
How to use the Radiator BTU Calculator
- Measure the room's length, width and ceiling height in metres.
- Select the room type, since bedrooms need less heat per cubic metre than living rooms or bathrooms.
- Pick the glazing option that best matches the room to adjust for heat loss through windows.
- Read the heat output in Watts and BTU/hr and choose a radiator rated at or just above that figure.
- For 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)
See it in your business.
Frequently 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 is the software UK heating businesses run on, with an assistant that answers every call and message, sends the quotes and books 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.
