Furnace BTU Calculator
Estimate the heating a home needs in BTU/h from its area, climate zone and insulation, and the furnace input rating for its efficiency.
Change the values and press Calculate to work out your own figures.
Enter the heated area, climate zone and insulation.
Show the working
- heat = area × 40–45 BTU per sq ft × 1
How to use it
Enter the heated area, climate zone and insulation.
Lengths can be decimals, fractions or tape entries such as 12' 6 1/2" (in feet or inches). Prices are optional.
Key facts
heat needed = area × BTU per sq ft for the zone × insulation; input rating = heat needed ÷ efficiency
Sizing
These are rules of thumb; a Manual J calculation accounts for windows, air leaks and the design temperature. Oversizing wastes fuel and shortens the furnace's life.
Questions
What size furnace for 2,000 sq ft?
80,000–90,000 BTU/h of output in a moderate climate.
Formulas
heat needed = area × BTU per sq ft for the zone × insulation; input rating = heat needed ÷ efficiency
Sources
Limitations
- Material densities are typical values (the midpoint of the commonly published range); real loads vary with moisture, grading and compaction, so check with your supplier or type in their density.
- Tank results are the geometric volume; real tanks have wall thickness and fittings.