How this calculator works
A room loses heat through its walls, windows, floor and ceiling and through draughts, roughly in proportion to its volume and to how much warmer it is inside than outside. The estimate starts from about 55 watts per cubic metre for a room kept at 21°C on a design day of −3°C (a temperature difference of 24°C), scales it by the temperature difference for the room type, and multiplies by factors for the glazing, the external walls and the insulation. A radiator is rated at ΔT50, meaning its mean temperature is 50°C above the room, and at ΔT30 it gives out only about (30 ÷ 50)^1.3, or 51%, as much.
Worked example: a 4 × 3.5 m living room with a 2.4 m ceiling is 33.6 m³. At 21°C with double glazing, one external wall and cavity insulation it needs 1,848 W (6,305 BTU/h). As a bedroom at 18°C it needs 1,617 W, with single glazing 2,218 W, with uninsulated solid walls 2,218 W, and as a corner room 2,125 W. On a low-temperature heat pump, the living room needs a radiator labelled 3,590 W at ΔT50. A 6 × 4.5 m lounge needs 3,564 W.
Heat needed for common rooms (double glazing, one external wall, 2.4 m ceiling)
Living rooms, bedrooms and bathrooms, in watts and BTU, with the size to look for on a heat pump.
| Room | Watts | BTU/h | Radiator on a heat pump |
|---|---|---|---|
| Small bedroom 3 × 2.5 m | 866 | 2,956 | 1,683 W |
| Double bedroom 4 × 3.5 m | 1,617 | 5,517 | 3,141 W |
| Living room 5 × 4 m | 2,640 | 9,008 | 5,129 W |
| Large lounge 6 × 4.5 m | 3,564 | 12,160 | 6,924 W |
| Kitchen 4 × 3 m | 1,386 | 4,729 | 2,693 W |
| Bathroom 2.5 × 2 m | 688 | 2,346 | 1,336 W |
Tips before you buy
- Choose a radiator whose output at ΔT50 is at or just above the heat the room needs, and avoid going far over. A radiator that is too big on a boiler can be turned down with a thermostatic valve.
- On a heat pump the flow temperature is lower (often 35 to 45°C), and a radiator then gives out roughly half its ΔT50 rating, so rooms often need bigger radiators or underfloor heating.
- A double panel with fins (type 22) gives more heat from the same width than a single panel, and a taller radiator gives more than a shorter one.
- Put the radiator under the window if you can: it offsets the cold down-draught from the glass.
- Better insulation and draught-proofing reduce the size of radiator you need, and the size of a boiler or heat pump.
Tools & materials for this project
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Frequently asked questions
What size radiator do I need for a living room?
A 4 × 3.5 m living room with double glazing and one external wall needs about 1,848 watts (6,305 BTU/h). Choose a radiator with that output at ΔT50, or a little more.
How many watts per square metre of floor?
There is no fixed figure, because the height, glazing and insulation change it. This room, with a 2.4 m ceiling, needs about 132 W per square metre of floor, more for single glazing or solid walls and less for a well-insulated room.
What is ΔT50?
It is the standard way of rating radiators: the average water temperature in the radiator is 50°C above the room temperature, which is roughly a flow of 75°C and a return of 65°C in a 20°C room.
Why do heat pumps need bigger radiators?
A heat pump runs at a lower flow temperature, so a radiator gives only about 51% of its ΔT50 output at ΔT30. The living room would need a radiator labelled about 3,590 W instead of 1,848 W.
How do I convert BTU to watts?
Divide the BTU per hour by 3.412. A radiator of 6,000 BTU gives out about 1,760 watts.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.