EngineerHub.orgDrawing No. EH–TH–036 // Thermal Engineering & HVAC
Radiator Sizing Calculator
Reviewed August 2026
Estimate room heat loss, correct radiator output for real water temperatures, and see the catalogue ΔT50 capacity you should select.
01 // Inputs
Radiator operating temperatures
02 // Results
ΔTmean = (Tsupply + Treturn)/2 − Troom.
Radiator correction uses Q/Q50 = (ΔTmean/50)n. The quick heat-loss method is deliberately a screening estimate; use a room-by-room heat-loss calculation for final selection.
Background & method
Radiator output depends on the temperature difference between the emitter and room. Manufacturer data commonly provides output at a reference ΔT, while low-temperature heat-pump systems need a substantial correction.
FAQ
It is the reference temperature difference commonly used to quote radiator output. Actual output falls as mean water temperature approaches room temperature.
Yes. Enter the lower supply and return temperatures. The correction factor will show why low-temperature systems often need larger radiators.
No. It is a screening estimate. Final emitter sizing should use room-by-room fabric, ventilation and design-temperature heat loss.