Furnace Running Cost Calculator
Estimate what your furnace costs to run from its heat output, AFUE efficiency and your own fuel price — natural gas, propane or heating oil.
Calculator
Delivering 480,000 BTU with a 95% furnace burns 5.05 therms ≈ $7.58 at $1.50 /therm (your rate — no tariff is stored).
A furnace burns fuel to make heat, but not all of the fuel’s energy reaches your rooms — the rest goes up the flue. The fraction that becomes useful heat is the AFUE (Annual Fuel Utilization Efficiency). Running cost is the heat you need, divided by that efficiency to get the fuel actually burned, priced at your own rate. The only wrinkle is that different fuels pack different energy per unit, so the tool switches the denominator when you pick gas, propane or oil.
Enter the total BTU delivered (capacity × hours, or read your heating load), the AFUE as a decimal, and your price per unit. Natural gas is priced per therm (100,000 BTU), while propane and heating oil are priced per gallon (91,500 and 138,500 BTU respectively). The stored numbers are stable physical energy contents — the price is always yours.
Formula
Fuel burned and its cost:
fuel units = heat delivered (BTU) ÷ (AFUE × energy content per unit)cost = fuel units × your price per unit
Energy content per unit (stable constants): natural gas 100,000 BTU/therm, propane 91,500 BTU/gallon, heating oil 138,500 BTU/gallon. Dividing by AFUE accounts for the flue losses — a 95% furnace burns about 16% less fuel than an 80% one for the same delivered heat.
Worked example
Delivering 480,000 BTU with a 95% AFUE gas furnace:
- therms = 480,000 ÷ (0.95 × 100,000) = 5.05 therms
- cost = 5.05 × $1.50 = $7.58
Switch the same job to propane at $2.50/gallon: gallons = 480,000 ÷ (0.95 × 91,500) = 5.52 gal, costing $13.80 — propane usually costs more per delivered BTU than pipeline gas. An 80% furnace instead of 95% would burn 6.0 therms for the same heat, about 19% more fuel.
Comparing fuels and staying safe
To decide whether gas is actually your cheapest heat, compare it against a heat pump and other fuels on delivered $/MMBTU using electric vs gas heat and $/MMBTU by fuel. To see the payback of a higher-AFUE furnace, use AFUE upgrade savings. For the furnace size behind the output figure, see furnace size.
This is a planning estimate: real fuel use varies with weather, distribution losses, cycling and how tight the house is. Read your meter over a known period to calibrate. Whatever the numbers say, gas work belongs to a licensed pro — a cracked heat exchanger or bad venting is a carbon-monoxide risk, so keep CO alarms installed.
One subtlety worth understanding: this tool works from the heat delivered, not the fuel input, and the AFUE bridges the two. A furnace with a 100,000 BTU/h input rating and 95% AFUE delivers about 95,000 BTU/h of usable heat, so if you read an input rating off the nameplate, multiply by AFUE before entering the delivered figure — otherwise you would double-count the efficiency and overstate the fuel. When in doubt, enter the delivered heat you actually need and let the tool add the flue losses back for you.
Reference table
Fuel burned to deliver 480,000 BTU at 95% AFUE (price your own units in the tool):
| Fuel | Energy content | Units burned | Sample price | Cost |
|---|---|---|---|---|
| Natural gas | 100,000 BTU/therm | 5.05 therms | $1.50/therm | $7.58 |
| Propane | 91,500 BTU/gal | 5.52 gal | $2.50/gal | $13.81 |
| Heating oil | 138,500 BTU/gal | 3.65 gal | $3.00/gal | $10.94 |