Conversions
- Therms = BTU ÷ 100,000. A therm is defined as 100,000 BTU.
- Propane gallons = BTU ÷ 91,500. This site uses 91,500 BTU/gallon as a round US planning factor. EIA HD-5 propane is 91,452 BTU/gallon (higher heating value).
Cost formulas
Main burner, per hour:
- If the efficiency derate is on: fuel BTU/hr = entered BTU/hr ÷ (efficiency ÷ 100). Default efficiency is 75% for vented units, labeled as an assumption.
- If the derate is off: fuel BTU/hr = entered BTU/hr (nameplate treated as fuel input).
- Natural gas $/hour = (fuel BTU/hr ÷ 100,000) × $/therm.
- Propane $/hour = (fuel BTU/hr ÷ 91,500) × $/gallon.
Day = hourly burner cost × hours/day, plus optional pilot. Month = day × days/month. Season = hourly burner cost × season hours, plus pilot × (season hours ÷ hours/day) days. A continuous pilot is 24 hours/day, not derated by fireplace efficiency.
Default US sample (no pilot)
30,000 BTU/hr, 4 hours/day, 30 days, 720 season hours, 75% efficiency assumption.
- Natural gas at $1.47/therm: $0.59/hour, $2.35/day, $70.56/month, $423.36/season.
- Propane at $2.67/gallon: $1.17/hour, $4.67/day, $140.07/month, $840.39/season.
Default rates
$1.47 per therm is a US residential planning figure in the typical EIA-range for 2025–2026. $2.67 per gallon is rounded from EIA residential propane around March 2026 (~$2.67). Edit the rate in the calculator to match your bill.
Sources
- Therm definition (100,000 BTU) — A therm is a heat-equivalent unit equal to 100,000 British thermal units.
- EIA propane heat content (HD-5) — EIA lists about 91,452 BTU per gallon of propane (HHV). This site uses 91,500 for round planning math.
- EIA residential natural gas prices — Default $1.47/therm is a US planning figure in the typical residential range — not your tariff.
- EIA residential propane prices — Default $2.67/gallon is rounded from EIA winter 2025–2026 residential propane (~$2.67 in Mar 2026).
Disclaimer
Estimates are unofficial and may be wrong for your house. They omit taxes, customer charges, tank rental, delivery fees, and any non-fuel costs. GasFireCost is not affiliated with fireplace manufacturers, dealers, or energy utilities. Do not treat results as safety, installation, or health guidance. Verify rates, therm factors, and appliance ratings with your utility or propane supplier and the unit’s rating plate.
Shareable URLs encode the same inputs as query parameters (fuel, BTU, hours, days, rate, pilot, efficiency, season hours) so a scenario can be reproduced without an account.