Furnace Sizing Calculator

Roughly 30 to 60 BTU of heating output per square foot, depending on climate. Zone 1 homes in the warm South need about 30 to 35 BTU per square foot, while Zone 5 homes in the far North need 50 to 60. Insulation, ceiling height and window area move the number inside that range.

Input BTU is the gas the furnace burns per hour. Output BTU is the heat that reaches your ductwork. The difference is the AFUE rating: an 80,000 BTU input furnace at 80% AFUE delivers 64,000 BTU of heat, while the same input at 96% AFUE delivers 76,800 BTU. Furnaces are sold by input, so size to output and then pick the input that gets you there.

In a moderate climate (Zone 3, 40 to 45 BTU per square foot) a 2,000 square foot house needs about 80,000 to 90,000 BTU of heating output. That is a 100,000 BTU input furnace at 80% AFUE or an 80,000 to 100,000 BTU input furnace at 96% AFUE. Colder zones push the number higher; a well insulated newer home pulls it lower.

No. An oversized furnace heats the house in short bursts, shuts off, and repeats. That short cycling wastes gas, wears out the igniter and blower faster, and leaves rooms far from the thermostat cold. The correct size runs longer, quieter cycles and holds a steadier temperature.

No. Manufactured homes use furnaces built to a different standard with their own sizing rules. Use the mobile home furnace sizing calculator instead.

It gets most homes within one furnace size and is a good starting point for shopping. For a final selection on a house with unusual windows, a finished basement, vaulted ceilings or an addition, a Manual J load calculation measures the actual heat loss room by room.