Furnace Size Calculator

In a moderate climate at 45 BTU per square foot, a 2,000 sq ft house needs roughly 90,000 BTU of output, which is about a 95,000 BTU input furnace at 95% AFUE. The same house needs about 65,000 BTU input on the Gulf Coast and about 130,000 in northern Minnesota. That spread is why square footage alone is only a screening number: it cannot see your window area, orientation, air leakage, or duct losses. Use it to sanity-check a quote, not to place an order.

No, and it is not a substitute for one. ACCA Manual J is the ANSI-recognized residential load standard: it models every window by size and orientation, wall and attic R-values, measured or assumed infiltration, duct losses in unconditioned space, and the 99% winter design temperature for your specific location. Manual J results frequently come in 20–40% below rule-of-thumb sizing on a well-sealed house. Any contractor replacing a furnace should run Manual J and select equipment with Manual S before quoting. Local mechanical code and a licensed HVAC contractor govern the final size — this calculator is a planning estimate only.

Input BTU is the fuel the burner consumes per hour, and it is the number on the model label and in the catalog. Output is the heat that actually reaches your ducts. AFUE (Annual Fuel Utilization Efficiency) is the seasonal ratio between them, so output ≈ input × AFUE. A 100,000 BTU input furnace at 80% AFUE delivers about 80,000 BTU; at 96% it delivers about 96,000. That is why this calculator computes the output your house needs first, then divides by AFUE to tell you which input rating to shop for.

An oversized furnace satisfies the thermostat in a few minutes and shuts off, so it short-cycles: temperature swings get wider, rooms far from the plenum never warm up because the blower stops before the ducts do their work, the burner is noisy, and the igniter, gas valve, and blower motor rack up starts they were not designed for. There is no efficiency prize either, because a furnace is least efficient during warm-up. ACCA Manual S caps furnace selection at 140% of the calculated design heating load for exactly this reason.

A single-stage furnace only runs at 100% fire, so it is the least forgiving of a sizing error. A two-stage unit runs a low fire near 65–70% of capacity for most of the season and only steps up in the coldest weather. A modulating furnace with a variable-speed blower ramps between roughly 35% and 100% in small increments, holding room temperature within about a degree and running long, quiet, even cycles. If your load lands between two common sizes, a two-stage or modulating unit at the larger size behaves far better than a single-stage one.