Radiator BTU Calculator
Calculate heat loss in BTUs and Watts for any room based on dimensions, window exposure, insulation levels, and exterior walls.
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How Room Heat Loss is Calculated
Sizing a radiator requires calculating the cubic volume of air that must be heated, adjusted for exterior wall exposure, heat transmission through windows, and wall insulation quality.
Base Heat Loss Formula
BTU/hr = Room Volume (cu ft) × Heat Factor × Exposure Multipliers
1 Watt = 3.412 BTU/hr. 1 Linear Foot of hydronic baseboard delivers ~550-600 BTU/hr at 180°F water.
Delta T50 vs. Delta T60 Standards
Modern European and British radiator ratings are certified at ΔT 50°C (mean water temp of 70°C in a 20°C room). Older boiler systems run hotter (ΔT 60°C / 180°F water), delivering approximately 20% higher output per panel.
Frequently Asked Questions
Is it better to slightly oversize or undersize a radiator?
Always slightly oversize. An undersized radiator will run continuously without reaching target comfort on sub-zero winter nights. Oversized radiators allow running lower, more efficient boiler water temperatures (ideal for modern condensing heat pumps and boilers).
Can I split the BTU load between two smaller radiators?
Yes, and in larger rooms (> 200 sq ft) splitting the load into two radiators installed on opposite walls produces far more balanced radiant heat and eliminates cold drafts.
Why do bathrooms require higher BTU ratings?
Bathrooms have higher target comfort temperatures (75°F / 24°C), higher ventilation air turnover, and cold tiled surfaces that absorb radiant body heat.