Proper furnace sizing is critical for comfort and efficiency. An oversized furnace short-cycles (turns on and off frequently), wasting energy and causing uneven temperatures. An undersized unit runs constantly and cannot keep up on the coldest days.
Professional HVAC contractors use ACCA Manual J to perform a detailed heat-loss calculation. This calculator provides a simplified estimate based on the same core factors: building volume, climate severity, insulation quality, window area, and air infiltration.
Climate zone determines the design temperature difference (delta-T) between desired indoor temp (68-70 °F) and the coldest expected outdoor temp. Insulation and home age affect the overall heat-loss coefficient. Windows are the weakest thermal link in most homes, losing 10-25x more heat per square foot than insulated walls.
Furnace nameplates show input BTU. A 96% AFUE furnace rated at 100,000 BTU input delivers 96,000 BTU of heat output. This calculator shows the output BTU your home needs; choose a furnace whose output meets or slightly exceeds this number.