Radiator BTU
heat loss calculator.
Enter the room dimensions, insulation level, and glazing — get heat loss in watts and BTU/hr with radiator sizing recommendations for gas boilers and heat pumps.
// BS EN 12831 · UK −3°C design temperature · ΔT50 radiator correction · K2 and K3 sizing
ROOM DIMENSIONS
INSULATION LEVEL
EXTERNAL WALLS
// walls exposed to outside, not adjacent rooms
NUMBER OF WINDOWS
// affects radiator output correction factor
CALCULATED HEAT LOSS
3,170 BTU/hr
DESIGN LOAD
heat loss + 20% safety margin
3,806 BTU/hr
HEAT LOSS BREAKDOWN
RADIATOR SIZING
×0.97 correction at 75/65°C
Single radiator
K2 600 × 600
1,169W
1200W rated
Two radiators
2× K2 600 × 400
1,559W
2× 800W rated
Based on UK −3°C external design temperature (BS EN 12831). For complex rooms, multiple heat emitters, or formal heat pump design, consult a Gas Safe or MCS-certified engineer.
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HOW IT WORKS
The calculator uses U-values and the BS EN 12831 fabric heat loss method. It sums losses through external walls, windows, ground floor, and exposed ceiling, then adds ventilation losses based on room type and air change rate. A 20% safety margin gives the design load.
RADIATOR CORRECTION
UK radiators are rated at ΔT50 — mean water temperature 70°C in a 20°C room. At lower flow temperatures the actual output drops significantly. The correction factor (n-factor 1.3 for panel radiators) adjusts the rated output to your system's actual operating temperatures.
HEAT PUMP SIZING
At 45/35°C a K2 radiator produces around 30% of its ΔT50 rated output. A 2000W radiator on a gas system needs to be roughly 3600W rated — or two radiators — on a heat pump. Getting radiator sizing right before installation avoids expensive remedial work after.
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