BTU calculator for a room

Work out how many BTU an hour a room needs to stay warm on a cold day, and what that means in watts and radiator size.

Method and tables from the MCS Heat Pump Calculator.

Your postcode, or its first lettersOnly the letters at the start are used, for the local design temperature.

Height above sea levelHigher ground is colder. Maps such as Ordnance Survey's show it.

Room

When was the home built?

Before 2000

2000 to 2006

After 2006, to current building regulations

Size of the room

LengthAlong the outside wall.

WidthInto the room.

Ceiling height

Which walls are outside walls?

One (the length)

Two, at a corner

Two, opposite each other (both lengths)

None

Outside walls are

Window areaWidth × height of each window, added together.

Windows are

Outside door area0 if the room has no outside door.

Above the ceiling

Another heated room

A loft, under a pitched roof

A flat roof

Below the floor

Another heated room

The ground: a solid floor

The ground: a suspended timber floor

Water in the radiatorsRadiators are sold by their output with the water 50°C warmer than the room (ΔT50).

75°C flow, 65°C return (a boiler set high: the usual radiator rating)

55°C flow, 45°C return (a condensing boiler turned down)

50°C flow, 40°C return (a heat pump on a cold day)

45°C flow, 35°C return (a heat pump run cooler)

No radiator: just the heat loss

This room needs about 2,124 BTU an hour (622 watts) to stay at 21°C on a cold day in LS postcodes.

BTU an hour

Heat loss	622 W
In BTU an hour	2,124 BTU/h
Radiator rating at ΔT50	622 W, 2,124 BTU/h

Air leaking in and out 391 W (63%)
Outside walls 100 W (16%)
Windows 132 W (21%)

Show the workings

Step	Sum	Answer
Temperature difference to design for	21°C room − -2.5°C outside	23.5 °C
Volume of the room	4 m × 3.5 m × 2.4 m	33.6 m³
Ventilation	1.5 air changes an hour × 0.33 × 33.6 m³ × 23.5°C	390.852 W
Outside wall, less windows and door	4 m × 2.4 m − 2 m²	7.6 m²
Outside walls	7.6 m² × 0.56 W/m²K × 23.5°C	100.016 W
Windows	2 m² × 2.8 W/m²K × 23.5°C	131.6 W
Heat the room loses on a design day	391 + 100 + 132	622.468 W
In BTU an hour	622 W ÷ 0.2930711 W per BTU an hour	2,123.949 BTU/h
Radiator rating needed at ΔT50	622 W, as the water is about 50°C above the room	622.468 W

What this leaves out or assumes

Walls to other heated rooms, and to neighbours, are taken to lose no heat. The MCS method allows for some loss through party walls and to unheated rooms, so a room with those needs a little more.
No extra is added for heating that goes off and on, an exposed site or a fireplace. An installer's full heat loss survey covers these.

Where the figures come from

Design temperature, lounge: 21 °C. Source: MCS Heat Pump Calculator, version 1.10 (MCS). Its tables come from CIBSE Guide A and the Domestic Heating Design Guide.. Checked 13 November 2026.
Outside design temperature, LS postcodes (E Pennines (Finningley)): -2.5 °C. Source: MCS Heat Pump Calculator, version 1.10 (MCS). Its tables come from CIBSE Guide A and the Domestic Heating Design Guide.. Checked 13 November 2026.
Air changes an hour, lounge, before 2000: 1.5 per hour. Source: MCS Heat Pump Calculator, version 1.10 (MCS). Its tables come from CIBSE Guide A and the Domestic Heating Design Guide.. Checked 13 November 2026.
Heat to warm the air, per cubic metre per degree: 0.33 W/m³K. Source: MCS Heat Pump Calculator, version 1.10 (MCS). Its tables come from CIBSE Guide A and the Domestic Heating Design Guide.. Checked 13 November 2026.
U-value: Cavity wall, brick both sides, 50 mm mineral wool in the cavity, plastered: 0.56 W/m²K. Source: MCS Heat Pump Calculator, version 1.10 (MCS). Its tables come from CIBSE Guide A and the Domestic Heating Design Guide.. Checked 13 November 2026.
U-value: Wood or PVC frame, double glazed: 2.8 W/m²K. Source: MCS Heat Pump Calculator, version 1.10 (MCS). Its tables come from CIBSE Guide A and the Domestic Heating Design Guide.. Checked 13 November 2026.
One British thermal unit (IT) per hour, in watts: 0.2930711 watts. Source: NIST Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically. Checked 13 November 2026.

BTU and watts

A BTU an hour is a unit of heating power, like the watt. One BTU an hour is 0.2930711 watts, so 1,000 watts is about 3,412 BTU an hour. Radiators and heaters in the UK are sold in both. The BTU to kW converter turns one into the other.

Why not just use the room's size?

Quick BTU rules work from the room's volume alone. But two rooms the same size can need very different amounts of heat: a room with a solid brick wall, big single-glazed windows and a loft above loses far more than one with insulated cavity walls and a heated room above. This calculator works out each wall, window, floor and ceiling separately, the way heating engineers do.

How the sum works

Each surface facing outside loses its area times its U-value times the temperature difference to a cold day where you live. Air leaking in and out adds more. The figures and method are the ones MCS uses for heat pump installations: see them all.

This works one room at a time. For a new boiler or heat pump, an installer's survey sizes the whole home. This gives information, not advice.

Work out something else

Radiator size calculator
The radiator output a room needs, in watts and BTU, by the MCS heat loss method.
BTU to kW converter
Convert heater and air conditioner output between BTU and kilowatts.