Propane Tank Hours Left Calculator
The gauge says "half." This says how many hours of heat that is, and when to call for the refill before the pilot light tells you the hard way.
Real numbers by appliance: furnace 80k–120k BTU/hr · wall furnace 8k–18k · patio heater 40k–45k · camp stove 8k–12k · generator 10k–24k · grill 20k–35k. The safe floor is 10%, running to empty airlocks the lines and relighting costs a service call.
How the math works
- Energy: 1 gal propane = 91,500 BTU. Usable = gallons × level% × 91,500.
- Hours = usable BTU ÷ (appliance BTU/hr × duty%). A 100 gal tank at 60% feeding a 100k BTU furnace at 70% duty = ~78 hours of furnace runtime ≈ 11 heating days.
- Safe floor: subtract the 10% reserve, propane deliveries are scheduled at 20%, and below 5% you're begging for a truck on the emergency rate.
- Propane costs ≈ $3.00–4.20/gal (2026 US); the per-heating-hour row converts that to the number that competes with electric baseboards and firewood.
How long does a 100 gallon propane tank last running a furnace?
At full capacity with a 100,000 BTU furnace cycling 70% of the time: ~130 hours of runtime, in a cold week (10 h/day of heat demand) that's about 13 days. A 60% fill is ~8 heating days.
Why not run the tank to empty?
Running out airlocks the gas lines, the furnace locks out and relighting usually takes a technician ($150+), and tanks under 5% risk vapor-lock in freezing weather. The 10% floor exists for the winter, not the summer.
How many hours does a 20 lb propane tank last on a heater?
20 lb ≈ 4.7 gal = 430k BTU. On a 40,000 BTU patio heater at full: ~10.7 hours. On a camp stove (10k BTU): ~43 hours of cooking.