Radiator BTU Sizing Calculator

Multiply the room volume (length times width times height) by a heat factor in BTU per hour per cubic foot that reflects the room's insulation and exposure, then the radiator must supply at least that many BTU per hour. This calculator computes room volume for you and applies the heat factor you choose, then also converts the result to watts.

The heat factor depends on insulation, windows, exterior walls, and climate, so a single fixed value would be misleading. Heating guides quote ranges roughly from 3 to 6 BTU per hour per cubic foot for well-insulated to poorly-insulated rooms. Use a value appropriate to your room from a qualified heating reference or installer.

Many radiators, especially in countries outside the United States, are rated in watts rather than BTU per hour. One watt equals 3.412142 BTU per hour, an exact unit conversion. This calculator divides the BTU per hour figure by 3.412142 to give the equivalent heat output in watts so you can match either rating.

Yes. Heating by volume captures ceiling height, which a floor-area method ignores. A room with a high or vaulted ceiling holds more air to heat and needs more output than a standard-ceiling room of the same footprint. That is why this calculator multiplies length, width, and height for the volume.

No. A formal heat-loss calculation (for example to a recognized engineering method) accounts for wall, window, roof, and air-change losses individually. This volume-times-factor approach is a quick sizing estimate. For a precise specification, use a recognized heat-loss procedure or consult a heating engineer.