Work out furnace sizing instantly with clear inputs, formula shown and shareable results.
A furnace is catalogued by input rating, but the building needs output. Adding a modest sizing margin to the design heat loss gives required output; dividing by AFUE converts that to the input rating to specify. Natural gas at roughly 1030 BTU per cubic foot then gives the appliance gas demand, and a 50 °F temperature rise gives the blower airflow.
Furnace input
Input = heat loss x (1 + margin) / AFUE
Blower airflow
CFM = output BTU/h / (1.08 x temperature rise)
Preliminary estimate only. Confirm appliance input, venting and gas supply capacity with a licensed installer against the applicable fuel gas code.
Input = heat loss x (1 + margin) / AFUE. A furnace is catalogued by input rating, but the building needs output.
Most guidance caps oversizing at 15-25 percent of design heat loss. Beyond that the furnace short-cycles and comfort and efficiency both fall.
This calculator takes 3 inputs: Design heat loss, Furnace AFUE, Sizing margin. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.