Radiator Size (BTU) Calculator
Work out how much heat a room loses on a cold day, room by room, and the radiator size to look for at your boiler or heat pump’s water temperature.
About this tool
Instead of a single “watts per cubic metre” rule, this adds up the heat lost through each part of the room on a cold day: outside walls, windows and doors, the floor and ceiling (only where there’s an unheated space or the ground beyond them), plus the warm air lost to draughts and ventilation. Each part is its area × a typical U-value for that construction × the difference between the room and outside temperatures. Room temperatures and air-change rates follow the usual UK design values, such as 21 °C for a living room, 18 °C for a bedroom and 22 °C for a bathroom, and you can change them.
Radiators are sold by their output at ΔT50, which assumes the water is on average 50 °C hotter than the room. Modern condensing boilers and especially heat pumps run cooler, so a radiator gives out less: at a heat pump’s ΔT25 or so, only about 40% of its ΔT50 figure. The tool corrects for this with the standard (ΔT ÷ 50)1.3 formula, so the “rating to look for” is the number to compare against catalogue figures. A watt is 3.412 BTU/h.
This is an estimate for choosing a radiator, not a full heat-loss survey to BS EN 12831, which a heat pump installer must do. Your figures are remembered on this device.