Heating load depends on where you live far more than cooling does — a square foot in Minnesota needs nearly double the BTUs of one in Georgia. This calculator applies standard climate-zone factors plus an insulation adjustment.
Home details
sq ft
Required heat output
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Furnace input size at 80% AFUE—
Furnace input size at 95% AFUE—
BTU per square foot used—
Furnaces are sold by INPUT BTU, but what heats your house is OUTPUT (input × efficiency). That’s why the two furnace rows are bigger than the top number.
The climate-zone method
The quick-sizing rule pros use for ballparking: multiply heated square footage by a climate factor — roughly 30–35 BTU/sq ft in the hot South up to 50–60 BTU/sq ft in the coldest states — then adjust for insulation quality. It won't replace a room-by-room Manual J, but it will keep you from buying a furnace two sizes wrong.
A 1,600 sq ft house in Ohio (moderate-cold, 42.5 factor) with average insulation needs about 68,000 BTU of output. An 80% furnace must be an 85,000 BTU input unit to deliver that; a 95% high-efficiency unit only needs about 72,000 input — one reason the efficient furnace can physically be smaller.
Input vs. output — the trap on the spec sheet
Furnace boxes advertise input BTU. A "100,000 BTU" furnace at 80% AFUE only puts 80,000 BTU into your ductwork; the rest goes up the flue. Always compare your required output against the unit's output rating, or divide by AFUE like this calculator does.
Frequently asked questions
How many BTU do I need to heat 1,000 square feet?
Anywhere from about 33,000 BTU in a hot climate to 55,000+ in a very cold one, assuming average insulation. Climate zone matters more than any other single factor for heating.
What is AFUE and why does it change the size?
AFUE is the furnace’s efficiency — the percentage of fuel that becomes usable heat. An 80% furnace wastes 20% up the flue, so it needs a larger input rating to deliver the same output as a 95% unit.
Should I round up or down between furnace sizes?
Slightly up in cold climates or drafty houses, and to the closer size otherwise. Heavily oversized furnaces short-cycle, heat unevenly, and wear faster — bigger is not safer beyond one size up.