calcs.homes
calcs.homes
Enter your home's heated area, climate, furnace efficiency, and how tight the house is, and this furnace size calculator estimates the output BTU you need and the input rating to shop for.
Estimate only — a Manual J heat-loss calculation is the right basis for a replacement furnace.
We multiply heated area by a climate heat-loss factor — about 25 BTU/sq ft in the mild South up to 45 in the coldest regions — and adjust for how tight the house is. That is the heat the furnace must actually deliver. Dividing by the AFUE efficiency gives the input rating on the nameplate: a 95% furnace has to burn about 67,000 BTU of gas to put out 64,000 BTU of heat. We round the input up to the nearest common size (40k, 60k, 80k, 100k, 120k).
Furnaces are one of the most oversized appliances in a house because installers pad the rule of thumb. An oversized furnace heats in short blasts, cycles frequently, and makes rooms feel drafty as temperatures swing. A Manual J heat-loss calculation, or matching the size of a furnace that kept the house comfortable for years, beats a generic multiplier.
Roughly 45,000–70,000 BTU of output depending on climate and insulation — often an 80,000 BTU input furnace at 90%+ AFUE once efficiency is factored in.
Input is how much fuel the furnace burns; output is the usable heat delivered. Output = input × AFUE. A 100,000 BTU input furnace at 80% AFUE delivers 80,000 BTU.
No. Size for your design temperature, not the rare extreme. An oversized furnace short-cycles all winter and only feels marginally better on the coldest night.