Furnace Size Calculator

How It Works

Furnace sizing is based on how much heat your home loses on the coldest day of winter. The outdoor design temperature is the coldest temperature your area typically reaches, not the absolute record low. Your furnace needs enough output BTU to replace that heat loss and maintain your desired indoor temperature. Furnaces are rated with two BTU numbers: input (the fuel burned) and output (the heat delivered). A 95% AFUE (Annual Fuel Utilization Efficiency) furnace converts 95 cents of every fuel dollar into heat. Oversizing a furnace causes short cycling, which wastes fuel and creates uneven temperatures. A properly sized furnace should run nearly continuously on the coldest day.