Radiator BTU calculator

Work out how much heat a room needs in watts and BTU an hour, and the radiator output to look for, including the bigger sizes needed for heat pumps.

Your room

About 2.8 m², our assumption of 20% of the floor

Or older walls with insulation added

Concrete laid on the ground

Heat the room needs on a cold day
2,520 BTU/h
740 W, to keep it at 21°C when it’s −3°C outside
Radiator output to look for
2,800 BTU/h
820 W at ΔT 50, with 10% added
If it runs at ΔT 40
3,720 BTU/h
1,090 W listed at ΔT 50
If it runs at ΔT 30
5,430 BTU/h
1,590 W listed at ΔT 50, typical for a heat pump

Where the heat goes
  • Outside walls310 W42%
  • Windows140 W20%
  • Floor150 W20%
  • Ceiling0 W0%
  • Air leaking out130 W18%

This is an estimate by the heat loss method in BS EN 12831-1 and the CIBSE Domestic Heating Design Guide (2026). Each outside wall, window, ceiling and floor loses its U-value times its area times the temperature difference. U-values are the RdSAP 10 defaults for England, plus MCS’s allowance for heat escaping at joints and edges (thermal bridging) of 0.15 W/m²K, or 0.08 for a new build. A solid ground floor uses the standard’s ground method: 1.45 times its U-value times its area times the gap between the room and 11.3°C, London’s annual mean temperature. A suspended timber floor, with air underneath, and a floor over a garage or other unheated space are counted against the outside temperature. Rooms are kept at CIBSE’s temperatures (22°C for bathrooms, 21°C for living rooms and 18°C elsewhere, which is 21°C in a new build) with −3°C outside, the design temperature for London; colder places such as Birmingham (−5.1°C) or Glasgow (−5.6°C) need roughly 10% more. We assume the air changes 0.5 times an hour, the minimum in the CIBSE guide’s Table 2-18, losing 0.33 W per cubic metre for each degree; draughty rooms lose more. Each outside wall counts as the average of the room’s length and width. Window area is our own assumption of 10%, 20% or 30% of the floor for small, average or large windows, unless you enter your own. It leaves out doors, heat lost to cooler rooms next door, chimneys and extractor fans. 1 W is 3.412 BTU an hour.

Radiators are listed by their output at ΔT 50 under BS EN 442, with the water about 50°C warmer than the room, such as 75°C flowing in and 65°C out. At ΔT 40 a radiator gives 0.747 of that and at ΔT 30 0.513 (Stelrad’s factors), so heat pumps and boilers set to a low flow temperature need bigger radiators. The radiator output adds 10% to the heat needed to help the room warm up from cold; that’s a rule of thumb, not part of the standard. For a new heating system, ask for a room-by-room heat loss calculation.

What goes in

Measure the room’s length, width and height in metres or feet. Count the walls that face outside, pick the glazing and roughly how much window there is, or type in the window area. Then say what the walls are made of and what’s below and above the room. These set the U-values, the heat that passes through each square metre for every degree of difference between inside and out.

What the result means

The heat needed is what the room loses at −3°C outside, with the room at 22°C for a bathroom, 21°C for a living room and 18°C for a bedroom, kitchen or hallway, which are 21°C in a new build. The default room, 4 m by 3.5 m with two outside walls, filled cavity walls and a solid floor, needs about 2,520 BTU an hour, or 740 W.

Radiators are listed at ΔT 50, with the water 50°C warmer than the room. Heat pumps and boilers set to a lower flow temperature run nearer ΔT 30, where a radiator gives about half its listed output. In England, a new or fully replaced wet heating system should be sized to work at a flow temperature of 55°C or lower.

The rules and figures behind it

This follows the heat loss method in BS EN 12831-1 as set out in the CIBSE Domestic Heating Design Guide (2026), with London’s −3°C design temperature, the RdSAP 10 default U-values for England and MCS’s allowance for heat lost at joints and edges. Solid ground floors are counted against London’s 11.3°C annual mean temperature, since the ground stays warmer than the air. It assumes the air changes 0.5 times an hour, the guide’s minimum, so draughty rooms need more. It adds 10% so the room can warm up from cold.

Many online BTU calculators give much bigger figures. Some use rules of thumb such as 40 W for each cubic metre. Others use older design figures, such as −1°C outside, a full air change every hour and ground floors that lose heat as if outside air were underneath. For a new boiler or heat pump, ask for a room-by-room heat loss calculation.

Sources

Radiator BTU calculator BTU calculator Radiator size calculator Heat loss calculator Radiator output Delta T Heat pump radiators Watts to BTU

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