What this calculator does
The furnace size calculator gives a general, rule-of-thumb estimate of the heating output (in BTUs per hour) a furnace should provide to comfortably heat your home.
The formula
Heating professionals commonly use a BTU-per-square-foot factor that scales with climate severity: roughly 30-35 BTU/sq ft in hot climates up to 50-60 BTU/sq ft in very cold climates. This calculator multiplies your home’s square footage by the factor for your selected climate zone, then adjusts for ceiling height above the standard 8 feet, since taller rooms hold more air to heat.
This is a simplified planning estimate, not a substitute for a professional Manual J load calculation, which accounts for insulation, window count, air sealing, and local design temperatures. Have an HVAC contractor confirm sizing before purchasing a furnace.