What this calculator does
Furnace size (BTU) works out heating capacity needed for a home, from floor area, climate band and insulation. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaBTU/h = floor area × climate BTU/ft² × sun factor × insulation factor ÷ (efficiency ÷ 100)
The inputs explained
| Field | What to enter |
|---|---|
| Floor area to heat (ft²) | A number, measured in ft². Starts at 1800. |
| Winter climate band | Choose from Mild, min. winter temp above 21°C / 70°F, Cool, 16-20°C / 61-68°F, Cold, 11-15°C / 52-59°F, Very cold, 6-10°C / 43-50°F, Severe, -4 to 5°C / 25-41°F, Extreme, -9 to -5°C / 16-23°F, Bitter, -19 to -10°C / -2 to 14°F, Arctic, -24 to -20°C / -11 to -4°F, Polar, below -25°C / -13°F. |
| Sun exposure | Choose from Sunny, Average, Shaded. |
| Insulation level | Choose from Heavily insulated, Average, Poorly insulated. |
| Furnace efficiency (AFUE) (%) | A number, measured in %. Starts at 95. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with floor area to heat
Every other input is held at the calculator’s starting values while floor area to heat varies. Select any row to load that scenario into the calculator.
How the answer changes with floor area to heat
| Floor area to heat (ft²) | Recommended furnace size | Calculated capacity | In kilowatts |
|---|---|---|---|
| 900 | 50,000 BTU/h | 47,368 BTU/h | 13.88 kW |
| 1,350 | 75,000 BTU/h | 71,053 BTU/h | 20.82 kW |
| 1,800 | 95,000 BTU/h | 94,737 BTU/h | 27.76 kW |
| 2,700 | 145,000 BTU/h | 142,105 BTU/h | 41.65 kW |
| 3,600 | 190,000 BTU/h | 189,474 BTU/h | 55.53 kW |
| 5,400 | 285,000 BTU/h | 284,211 BTU/h | 83.29 kW |