How long will your propane tank last?

How this works

Propane appliances are rated in BTU per hour (BTU/h): how much heat energy they burn at full flame. A tank holds a known amount of energy. Divide one by the other and you get the run time.

• Daily use (BTU) = Σ BTU/h × hours a day × flame-on %.
• Usable propane = tank contents × number of tanks × how full (%).
• Run time (days) = usable BTU ÷ daily BTU. We round the days and hours down, so the answer errs on the short side.
• Pounds per day = daily BTU ÷ 21,569. Gallons per day = daily BTU ÷ 91,452.

The constants

• 91,452 BTU per US gallon of propane, from the US Energy Information Administration.
• 4.24 lb per gallon at 60 °F, from Propane 101. That gives 91,452 ÷ 4.24 ≈ 21,569 BTU per pound. You will often see about 21,500 to 21,600 quoted; the difference is under 1%.

Tank sizes and the 80% rule

Propane expands when it warms up, so tanks are only filled to about 80% to leave room. Portable cylinders sold by the pound (1, 5, 11, 20, 30, 40 and 100 lb) are named for the propane they hold when properly filled, so a 20 lb cylinder holds about 20 lb, or 4.7 gallons. Permanently mounted under-mount (ASME) tanks are usually listed in gallons of water capacity, so the calculator counts 80% of that number. If you know the actual gallons or pounds in your tank, choose one of the “Other” options instead.

The starting example is one full 20 lb cylinder (about 431,377 BTU) running a Propex HS2000 heater at 7,200 BTU/h for 8 hours with the flame on half the time (28,800 BTU), plus a 2-burner stove at 14,500 BTU/h for 45 minutes (10,875 BTU). That is about 39,700 BTU, or 1.84 lb, a day, so the cylinder lasts about 10 days and 20 hours.

Why “flame on %” matters

A heater set to a comfortable temperature doesn’t burn all the time. Its thermostat lights and stops the flame. The flame-on share depends on how cold it is outside, your insulation and the thermostat setting. In mild weather it might be a quarter of the time; on a freezing night it could be most of the night. If you are not sure, use a higher number: the estimate will be shorter, which is the safe side.

Propane safety

• Alarms. Fit a carbon monoxide alarm and a propane (LP gas) leak alarm, test them regularly and replace them when they expire. Propane is heavier than air, so LP alarms go low down; follow the alarm maker’s instructions.
• Unvented heaters. Portable heaters such as the Mr. Heater Buddy and catalytic heaters release their exhaust into the van. Mr. Heater’s manual says never to operate the Buddy while sleeping and requires fresh-air openings; the US Consumer Product Safety Commission warns never to use fuel-burning portable heaters while sleeping in tents, campers or vehicles. Always follow the manufacturer’s guidance.
• Vented heaters. Heaters with an outside flue, like the Propex or an RV furnace, keep exhaust outside, but only when installed to the manufacturer’s instructions. Keep exhaust and intake clear of snow.
• Cooking. Never use a stove or oven to heat the van, and open a window or roof vent when cooking.
• Leaks. If you smell gas (a rotten-egg smell), turn the tank off, put out flames, don’t switch anything electrical, get out and air the van. Have the system checked before using it again.
• CO symptoms can feel like the flu: headache, dizziness, nausea, sleepiness or confusion. Get to fresh air at once and call emergency services.

This calculator is a planning estimate, not a safety device. Have propane systems installed and checked by a qualified technician.

Sources

• US Energy Information Administration, British thermal units: 1 gallon of propane = 91,452 BTU.
• Propane 101, propane facts: 4.24 lb per gallon at 60 °F.
• Paraco Gas, the 80 percent rule: why tanks are filled to about 80%.
• US Consumer Product Safety Commission, portable heaters and camping equipment: carbon monoxide warnings.
• Mr. Heater Portable Buddy (MH9BX) manual: 4,000 and 9,000 BTU/h, ventilation and “never operate while sleeping”.
• Appliance ratings: Propex Heatsource brochure (HS2000, 2.1 kW input); Suburban furnace service manual (SF-20, 20,000 BTU/h); Furrion 2-burner cooktop (8,000 + 6,500 BTU); Furrion 17-inch range (7,100 BTU oven); Norcold N64X/N84X specifications (1,500 BTU/h); Suburban SW6D water heater (12,000 BTU); Girard GSWH-2 (42,000 BTU); Camco Olympian Wave-3 (3,000 BTU).