Radiator Size Calculator

What size radiator a room needs: heat lost through walls, windows, roof, floor and air in watts and BTU, rated at ΔT50 or for a heat pump.

A radiator has to put out as much heat as the room loses on a cold day. This calculator works that out from the room's size, its outside walls, windows, roof and floor, and the air that leaks in, using typical U-values and CIBSE's design temperatures for each room. Radiator outputs are quoted at ΔT50, with the water about 50 °C warmer than the room. If your boiler or heat pump runs cooler, you need a bigger radiator: at ΔT30 a radiator gives about 51% of its rated output. A 4.5 × 3.5 m living room with two outside walls of unfilled cavity brick, 2.5 m² of double glazing and an uninsulated solid floor loses about 1,400 W (4,773 BTU/h) at −3 °C outside. So it needs radiators rated about 1,400 W at ΔT50, or about 2,720 W for a heat pump running at ΔT30.

Units

Room

Room length

Room width

Ceiling height

Length of outside wall

Window area

m²

Room temperature

°C

Coldest outside temperature

°C

Outside walls

Windows

Above the room

Below the room

Air changes per hour

Heating runs at

Heat needed: 1,399 W
Radiator output to choose: 1,399 W
Biggest heat loss: Outside walls

How it was worked out

Temperature difference: 21 °C − (-3 °C) = 24 K
Outside walls: (8 m × 2.4 m − 2.5 m² of glazing) = 16.7 m² × U 1.5 × 24 = 601 W
Windows and glazed doors: 2.5 m² × U 1.4 × 24 = 84 W
Floor: 15.75 m² × U 0.7 × 24 = 265 W
Air: 0.33 × 1.5 air changes × 37.8 m³ × 24 = 449 W
Total: 1,399 W = 4,773 BTU/h

The U-values are typical figures for each kind of wall, window, roof and floor; your home's real values depend on how it was built. Room temperatures are CIBSE's design figures for UK homes built before about 2007 (newer homes are designed for 21 °C in most rooms). The air change rates are typical of older, draughtier homes; a well-sealed modern home leaks far less air, so try a lower figure. For the coldest outside temperature, use your area's winter design temperature, the cold the heating should still cope with: about −3 °C for London. This is a simplified room heat loss; to size a boiler or heat pump for a whole home, installers do a full room-by-room calculation (BS EN 12831 in the UK and Europe, Manual J in the US). Gas work must be done by a registered engineer (Gas Safe in the UK).

Common use cases

Choose a new or replacement radiator for a room
Check whether existing radiators are big enough before fitting a heat pump
Compare a room's heat loss before and after insulation or new windows
Convert a room's heat need between watts and BTU

How to use

Choose metres and °C, or feet and °F.
Pick the room type, which sets its usual temperature and air change rate.
Enter the room's length, width and ceiling height, the length of outside wall and the window area.
Choose the walls, windows, what's above and below the room, and how hot your heating runs.
Read the radiator output to look for, at ΔT50 as brochures quote it.

FAQ

What does ΔT50 mean?

ΔT is the difference between the radiator's average water temperature and the room. Radiators are rated at ΔT50 under EN 442: for example water at 75 °C going in and 65 °C coming out, averaging 70 °C, in a 20 °C room. At lower temperatures the output falls by (ΔT ÷ 50)^1.3: about 75% at ΔT40 and 51% at ΔT30.

What size radiator do I need for a bedroom?

A 3.5 × 3 m bedroom with a 2.4 m ceiling, one 3.5 m outside wall of unfilled cavity brick with 1.5 m² of double glazing, a loft with 270 mm of insulation above and a heated room below needs about 471 W (1,608 BTU/h) at 18 °C, with −3 °C outside. For a heat pump running at ΔT30, that's a radiator rated about 916 W at ΔT50.

Why do heat pumps need bigger radiators?

Heat pumps work most efficiently with cooler water, often around 45 to 50 °C, so the radiators run at ΔT30 or less rather than ΔT50. At ΔT30 a radiator gives about half its rated output, so it needs to be roughly twice the size to heat the same room.

How do I convert watts to BTU?

Multiply watts by 3.412: 1,000 W is 3,412 BTU/h. To convert BTU/h to watts, divide by 3.412.

Which outside temperature should I use?

Use your area's winter design temperature: the cold the heating should still cope with. CIBSE puts London at about −3 °C, with colder figures for Scotland and higher ground and milder ones for the south-west. A colder figure gives a bigger, safer answer.

Results are estimates for planning, worked out from the figures you enter and typical values (U-values, air leakage, design temperatures) that vary from home to home. They are not engineering or professional advice. Have heating and cooling systems sized and fitted by a qualified installer: gas work must be done by a registered engineer (Gas Safe in the UK), and air conditioning refrigerant work by a licensed or certified technician.