About this tool
The Furnace AFUE Calculator is a free, in-browser tool that turns a furnace's input rating into usable heat output and compares the annual fuel cost of two efficiency levels. AFUE (Annual Fuel Utilization Efficiency) is the fraction of fuel energy delivered as heat over a season, so usable output = input BTU/hr × AFUE and the wasted portion is input × (1 − AFUE). An 80,000 BTU/hr furnace at 80% AFUE delivers 64,000 BTU/hr of heat; the same input at 96% delivers 76,800.
All calculations run locally in your browser and nothing you enter is stored. To estimate annual cost the tool uses the heat actually delivered over the season (output × running hours) and divides by the AFUE to get fuel energy consumed, then prices it by fuel: natural gas at roughly 100,000 BTU per therm, propane at about 91,500 BTU per gallon, and heating oil at about 138,500 BTU per gallon. Seasonal running hours vary by climate, so mild, moderate, and cold presets are provided and can be overridden.
Enter a comparison AFUE to see the annual savings of an upgrade, since the cost difference equals the annual delivered heat times the fuel price per BTU times (1/AFUE_old − 1/AFUE_new). Add the upgrade cost to get a simple payback in years. The classic 80%-to-96% jump cuts fuel use by about one-sixth for the same heat — meaningful in a cold climate with expensive fuel, marginal in a mild one. Figures are estimates; real bills depend on weather, sizing, and distribution losses.