Heating BTU per square foot
The heat a house loses on the coldest days depends on the climate and how well it is built. As a rule of thumb, this calculator uses these values for average insulation, then −20% for a well-insulated home or +25% for an old, drafty one:
| IECC climate zone | BTU/h per ft² |
|---|---|
| Zones 1–2 (hot) | 15 |
| Zone 3 (warm) | 25 |
| Zone 4 (mixed) | 32 |
| Zones 5–7 (cold) | 40 |
Worked example
A 1,800 ft² house in zone 4 with average insulation: 1,800 × 32 = 57,600 BTU/h (16.9 kW), or 4.8 tons, so a 5-ton heat pump. In cold zones the heat pump must deliver that output at the design temperature, so compare against the capacity at 5 °F, not the nominal rating.
Heat pump vs furnace sizing
Heat pumps are sold by cooling tons; their heating output drops as it gets colder. Furnaces are sold by input BTU: a 95% AFUE furnace with 60,000 BTU input delivers about 57,000 BTU/h. For permits and rebates, a Manual J calculation is the standard.
Frequently asked questions
What size heat pump do I need for 2,000 sq ft?
In zone 4 with average insulation about 64,000 BTU/h (5.3 tons); a well-insulated home about 51,000 BTU/h. Always check the output at your design temperature.
How many BTU per square foot to heat a house?
Roughly 15 BTU/ft² in hot climates to 40 BTU/ft² in cold ones for an average home; well-insulated homes need less.