Radiator and BTU calculator

What it works out

This estimates how much heat each room loses on a cold day, in watts and BTU/h, and the radiator output that keeps it warm. Add one room or several: you get each room's figure and a radiator size for it, which is what you choose radiators by.

It's an estimate to compare radiators with, not a heating design. It sizes no boiler, pipework or heat pump. Boilers and anything on gas are for a registered engineer (see our gas guidance), and a heat pump for a grant needs an installer's heat-loss survey.

How it works

Heat leaves a room two ways. Through the fabric: for each external wall, window, external door, a ceiling with a roof or loft above, and a floor on the ground, the calculator multiplies its U-value by its area and by the difference between inside and outside (ΔT). Walls lose their windows and doors first. Walls and floors between heated rooms are left out.

And in air: 0.33 × the room's air changes an hour × its volume × ΔT. Air changes go by room type and age: a bathroom or kitchen changes its air more than a bedroom, and a home built since 2000 is more airtight.

ΔT is the room's temperature less the outside design temperature. Rooms start at the usual design figures (21 °C for living rooms and studies, 22 for bathrooms, 18 for bedrooms, kitchens and halls, after CIBSE's Domestic Heating Design Guide), and the outside at a typical figure for the place you pick. U-values are typical published figures for the construction you choose. A margin covers heating that goes off and on. One watt is 3.412 BTU/h.

Radiators are rated at ΔT50: the water on average 50 °C warmer than the room, as with a boiler. At a lower ΔT a radiator gives (ΔT ÷ 50)^1.3 of its rating, so a heat pump at ΔT30 gets about half. The calculator divides the room's heat by that and rounds up to the next 100 W, splitting it over two or more radiators above 3,000 W.

A worked example

A ground-floor living room 4.5 m by 3.6 m, 2.4 m high, with two external walls (filled or insulated), 2.52 m² of older double glazing, a solid floor, a heated room above, a boiler, and -3 °C outside.

1. Room 1, living room: 4.5 m × 3.6 m, 2.4 m high: floor 16.2 m², volume 38.88 m³
2. 21 °C inside, -3 °C outside: a difference (ΔT) of 24 °C
3. 2 external walls: 8.1 m × 2.4 m = 19.44 m², less 2.52 m² of windows and doors = 16.92 m²
4. Walls: 16.92 m² × U 0.5 W/m²K × 24 °C = 203 W
5. Windows: 2.52 m² × U 2.8 W/m²K × 24 °C = 169 W
6. Floor: 16.2 m² × U 0.7 W/m²K × 24 °C = 272 W
7. Air: 0.33 × 1.5 air changes an hour × 38.88 m³ × 24 °C = 462 W
8. Room 1, living room: 645 W through walls, windows, floor and roof + 462 W in air = 1,106 W, plus a 10% margin = 1,217 W (4,153 BTU/h)
9. With a boiler, radiators give their rated output (ΔT50): 1,217 W → a radiator of at least 1,300 W, rounded up
10. The room needs 1,217 W × 3.412 = 4,153 BTU/h. That's their heat loss, not a boiler or heat pump size, which also allows for hot water and the system.
11. An estimate from typical figures, not a heating design. A heat pump for a grant (in the UK, under MCS) needs an installer's room-by-room heat-loss survey, and a boiler and anything on gas are for a registered gas engineer.

Common mistakes

• Counting internal walls as external. Only walls with the outside behind them lose heat here. A wall onto a neighbour's heated home counts as internal.
• Sizing a heat pump's radiators at ΔT50. A radiator's label gives its output at ΔT50. With a heat pump's cooler water it gives about half, so the room needs a bigger radiator, or two.
• Adding the rooms up into a boiler size. A boiler or heat pump also covers hot water and the system's own losses, and is chosen by an engineer or installer. The rooms' total is only theirs.
• Ignoring the windows. Glass loses heat several times faster than a filled wall. Measure the windows, frames included.
• Leaving out the ceiling under a cold loft. A top-floor room loses heat upwards. Say there's a roof above, and choose the loft's insulation.

Questions people ask