Furnace Efficiency Cost Calculator
A furnace efficiency cost calculator shows how much fuel a furnace burns to deliver the heat a home needs, and what that fuel costs, based on the furnace's efficiency rating. That rating is the annual fuel utilization efficiency, or AFUE, the fraction of fuel energy a furnace turns into useful heat over a season. An 80 percent AFUE furnace wastes a fifth of the fuel up the flue, while a 95 percent unit wastes only a twentieth. This tool divides the heat you need by the AFUE to find the fuel energy that must be purchased, then multiplies by your fuel rate to give the cost. Every input is editable so you can compare two efficiency levels, model any heat load, and use your own fuel price, whether in therms, gallons or another unit. The US Department of Energy sets minimum AFUE standards and publishes guidance on furnace efficiency and home heating cost. Use the result to weigh the running-cost savings of a high-efficiency furnace against its higher purchase price, or to budget for a heating season. Every figure is computed deterministically from the formula shown below, with a worked example that reconciles exactly to the calculator so you can follow each step yourself.
Fuel used divides the heat needed by efficiency, then prices it: cost = (heat / AFUE) x rate. Needing 80 therms of heat at 95% AFUE burns 84.21 therms, costing $126.32 at $1.50 per therm.
Furnace efficiency cost formula
Fuel used = H / AFUE
Cost = Fuel used x R
H = useful heat needed (therms)
AFUE = annual fuel utilization efficiency (fraction)
R = fuel rate (dollars per therm)
Dividing by AFUE accounts for the fuel lost as waste heat. A higher AFUE means less fuel is burned for the same useful heat, lowering the cost.
Worked example
A home needs 80 therms of useful heat from a 95 percent AFUE furnace, with fuel priced at $1.50 per therm.
- Fuel used = 80 / 0.95 = 84.21 therms
- Rate = $1.50 per therm
- Cost = 84.21 x 1.50 = $126.32
The furnace burns 84.21 therms costing $126.32. These are the calculator's default inputs, so the result above matches the widget exactly.
Furnace Efficiency Cost Calculator: frequently asked questions
What is AFUE?
AFUE stands for annual fuel utilization efficiency. It is the fraction of fuel energy a furnace converts into useful heat over a heating season, expressed as a percentage. A 90 percent AFUE furnace delivers 90 cents of heat for every dollar of fuel, with the rest lost mainly up the flue.
How much does a higher AFUE save?
The fuel saving is proportional to the change in efficiency. Going from 80 to 95 percent AFUE cuts fuel use by about 16 percent for the same heat, because you divide by a larger number. Whether that pays back the higher furnace price depends on your heat load and fuel price, which this calculator lets you test.
What units can I use?
Any consistent pair. If you enter heat needed in therms and the rate in dollars per therm, the cost is correct. You can also work in gallons of oil or propane, or in BTU, as long as the heat and rate share the same unit. Keep the AFUE as a fraction between 0 and 1.
Does AFUE include electricity for the blower?
No. AFUE measures the efficiency of converting fuel into heat, not the electricity the furnace's blower and controls use. For a full operating cost, add the blower's electricity cost, which you can estimate with an appliance energy cost calculator.
What is the minimum AFUE allowed?
The US Department of Energy sets minimum AFUE standards for new furnaces, which have risen over time and vary by furnace type and region. New gas furnaces are typically at least 80 percent, with high-efficiency condensing models reaching 90 to 98 percent.
Official sources
- Furnace efficiency (AFUE) standards and heating cost guidance: US Department of Energy (DOE). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.