Radiator Calculator Pro

Room-by-room radiator sizing. Watts and BTU/h output for any room, factoring volume, glazing, external walls, roof and floor exposure, and insulation standard.

Calculation
Heat output per room with adjustments for:
• Room type (8 presets: bedroom, living room, bathroom, kitchen, hallway, office, conservatory, garage)
• Internal volume
• Window-to-floor ratio
• Number of external walls (0–4)
• What's above (loft, pitched roof, flat roof, heated space)
• What's below (garage, unheated cellar, uninsulated floor, heated floor)
• Insulation level: poor (older single-glazed) → average (modern build) → good → excellent (new-build, triple-glazed)
• Mean water temperature - ΔT30, ΔT35, ΔT40, ΔT50, ΔT60

ΔT correction uses the standard panel-radiator exponent (n = 1.30) per EN 442 testing convention. A 5% safety margin is applied to avoid undersizing.

Heat-pump systems

Select ΔT35 or ΔT30 to spec radiators for low-temperature systems. Output is rescaled — a 1000W ΔT50-rated radiator delivers ~630W at ΔT35 — so you size to actual delivered heat, not nameplate.

Output
• Watts and BTU/h side-by-side
• Metric or imperial dimensions
• Whole-home heat-loss total
• Share or export room specs as text

For plumbers, heating engineers, and heat-pump installers sizing radiator schedules, and homeowners or landlords running their own jobs.