Heating Load Calculator (BTU)

It estimates the heating capacity needed to keep a room warm in winter — in BTU/hr, tons and kW. Enter the room size in sq ft (m²), insulation quality, glazing and how cold your winters get, and it returns the load plus the next standard furnace or heat pump size that covers it.

It starts from a rule of thumb of about 30 BTU/hr per square foot of floor (≈100 W/m²) for an averagely insulated room with a standard 8 ft (2.4 m) ceiling, scaled for taller rooms. That base is adjusted for insulation (roughly −30% for good, +30% for poor), winter severity, and large glazed areas. The calculator works in BTU/hr throughout, at 3.412 BTU per watt.

Around 20–25 BTU/hr per square foot (60–80 W/m²) for a well-insulated modern room, about 30 BTU/hr per square foot (100 W/m²) for average construction, and 40 BTU/hr per square foot (130 W/m²) or more for older, poorly insulated buildings or very cold climates. Tall ceilings, big windows and exposed walls all push the figure up — which is exactly what the calculator adjusts for.

They all measure the same thing — heat output. One kilowatt is about 3,412 BTU/hr, and a ton is 12,000 BTU/hr, so a 2,000 W panel heater delivers roughly 6,800 BTU/hr and a 12,000 BTU/hr (1 ton) heat pump delivers about 3.5 kW of heating. Electric resistance heaters are sold in watts; furnaces and heat pumps in BTU or tons — the calculator shows all of them.

Round up to the next standard size. For a heat pump note that its heating output falls just when you need it most — on the coldest days — so in cold climates size from the unit’s rated capacity at low outdoor temperature, not the nameplate figure. A heat pump also delivers 3–4 kW of heat per kW of electricity, so it costs far less to run than an electric resistance heater or furnace strip of the same output.

It is a sizing guide, not a heat-loss study. A formal calculation (EN 12831, Manual J or similar) works from the U-values of every wall, window, floor and roof plus air leakage and local design temperatures. For a typical room the rule of thumb lands close; for whole houses, extensions or unusual construction, have a full heat-loss calculation done.