Heating is sized by how fast a room loses heat in winter, and the quickest estimate is per square foot of floor. Start near 30 BTU/hr per square foot for average construction, then adjust up or down for insulation, windows and how cold your winters get. Round up to the next standard furnace, heater or heat pump.
The rule of thumb
For an averagely insulated room with a standard 8 ft (2.4 m) ceiling, budget about 30 BTU/hr per square foot (≈100 W/m²). Then scale it to your building:
| Construction | BTU/hr per sq ft | W/m² |
|---|---|---|
| Well-insulated, modern | 20–25 | 60–80 |
| Average | 30 | 100 |
| Older, poorly insulated / very cold | 40+ | 130+ |
Tall ceilings, large glazed areas and lots of exposed exterior wall all push the figure up.
Worked example
A 300 sq ft (28 m²) living room of average construction: 300 × 30 = 9,000 BTU/hr. In other units that is 9,000 ÷ 3,412 = about 2.6 kW, or 9,000 ÷ 12,000 = 0.75 tons. Round up to the next standard unit — a 3 kW heater or a 12,000 BTU (1 ton) heat pump.
Heating Load Calculator
Enter the room size, insulation, glazing and winter climate — it returns the load in BTU/hr, kW and tons, plus the furnace or heat pump size to install.
BTU, kW and tons — same thing
They all measure 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 1-ton heat pump delivers about 3.5 kW of heat. Resistance heaters are sold in watts; furnaces and heat pumps in BTU or tons.
Heat pumps need extra care in the cold
A heat pump’s output falls exactly when you need it most — on the coldest days. In a cold climate, size from the unit’s rated capacity at a low outdoor temperature, not the nameplate figure. The payoff: a heat pump delivers 3–4 kW of heat per kW of electricity, so it costs far less to run than an electric resistance heater or a furnace strip of the same output.
When to get a full calculation
This is a sizing guide, not a heat-loss study. A formal calculation (Manual J, EN 12831 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 proper heat-loss calculation done.