Propane vs Natural Gas Cost Calculator

By converting both to delivered BTU at the same furnace efficiency. One US gallon of HD-5 propane holds 91,452 BTU of chemical energy (NIST higher heating value), and one therm of natural gas holds exactly 100,000 BTU (the utility-standard definition). A therm of gas therefore delivers about 9.4 percent more heat than a gallon of propane, so the same furnace burns about 1.09 gallons of propane for every therm of gas it would have burned to keep the house warm. The calculator divides your annual heating load by the per-unit BTU and the AFUE for each fuel, then multiplies by your local price. The same load and the same AFUE apply to both sides, which is what makes the comparison fair.

Natural gas often costs less per delivered BTU where a gas connection already exists, but current delivered prices, fixed service charges, equipment, and availability can reverse a household comparison. Enter all-in local therm and gallon prices, including recurring charges, before comparing fuels.

Propane can make sense where gas service is unavailable or the cost to connect is high. Compare delivered fuel price, fixed charges, tank fees, equipment, and realistic annual use. The calculator compares fuel operating costs only, not connection or installation cost.

Propane and natural-gas retail prices can move differently because fuel supply, delivery, utility tariffs, fixed charges, contracts, weather, and local competition differ. There is no fixed amount or timing for either fuel's price changes. Enter current all-in delivered prices, then compare the calculator's break-even result with any supplier quote or utility tariff before signing a contract.

AFUE changes the fuel needed for the same delivered heat. When the calculator applies the same AFUE to both fuels, the break-even fuel-price ratio stays the same. A real replacement decision also needs equipment cost, venting, maintenance, comfort, and local fuel tariffs.