Heating guide
Radiator BTU calculator
Estimate the heat output required for a room, then use the result as a starting point when selecting a radiator.
Selection guidance
Understanding the estimate
How is this quick estimate calculated?
The calculator multiplies room volume by a room-type allowance in watts per cubic metre, then applies broad insulation and external-wall adjustments. It is useful for an initial comparison, but it is not a substitute for a standards-based heat-loss survey.
What does BTU mean?
BTU/h is a measure of heat output per hour. Radiator specifications may also show watts; 1 watt is approximately 3.412 BTU/h.
Should I choose the exact calculated output?
Treat the result as a baseline. A heating professional should confirm room heat loss and system operating temperatures before final selection.
Does lower water temperature affect radiator size?
Yes. A radiator produces less heat at lower flow and return temperatures, so a larger emitter may be needed. Use the manufacturer’s correction data for the intended system conditions.