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Free tool

Radiator BTU calculator

Work out how much heat a room needs and what size radiator to buy, for a gas boiler or a heat pump. Free, with no sign-up.

Which room?

This sets the temperature the room is designed to reach.

Room size
m
m
m
How many walls face outside?
What are the outside walls?
Windows and glazed doors

Total glass area. A typical sash window is about 1.5 m².

m²
What glazing?
What's above the room?
What's below the room?

This room needs about

4,900 BTU/h

1,440 watts at 21 °C inside, −3 °C outside

With a gas boiler
5,050 BTU
Radiator rated output (ΔT50)
With a heat pump
14,750 BTU
ΔT50 rating needed at 45 °C flow

Where the heat goes

  • Walls32%
  • Windows12%
  • Ceiling or roof0%
  • Floor22%
  • Draughts and ventilation35%

A guide for one room using typical values, not a full heat loss survey. Radiators are sold by their output at ΔT50; choose one at or above the figure shown. Heat pumps run cooler, so radiators need to be larger.

Planning a new boiler or heat pump?

We calculate every room before recommending a system. Send this room to us and we'll take it from there, or try the heat pump estimate.

Send this to Greens

We'll look at your room and get back to you about radiators or a full heat loss survey.

Your answers are sent with your details. See our privacy notice.

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