Field calculator
Propane tank runtime calculator
Knowing how long a propane tank lasts drives the refill schedule and whether a tank can even keep up with a burner, and the math is the usable tank energy divided by the appliance load. The energy is the tank gallons times about 91,500 BTU per gallon times the usable fraction. Enter the tank size in gallons, the appliance load in BTU per hour, and the usable percent. Common sizes help: a 20-pound grill cylinder holds about 4.6 gallons, a 100-pound cylinder about 23.6 gallons, and a 500-gallon tank is filled to roughly 400. Two real-world limits shape the answer. A tank is only filled to about 80 percent to leave room for the liquid to expand, so the usable amount is less than the nominal size. And in cold weather the liquid propane vaporizes more slowly, so a small cylinder cannot supply a large burner continuously, frosting up and starving the appliance well before it is empty, which is a sizing problem the runtime number alone does not show. Use this for planning refills and sanity-checking tank size, and confirm the tank, the regulator, the cold-weather vaporization rate, and the appliance BTU draw with the propane supplier.
Result
Propane tank runtime: hours = usable tank energy divided by the appliance load, where the energy is the tank gallons times about 91,500 BTU per gallon times the usable fraction. Enter the tank size in gallons, the appliance load in BTU per hour, and the usable percent. Common tank sizes: a 20-pound grill cylinder holds about 4.6 gallons, a 100-pound cylinder about 23.6 gallons, and a 500-gallon tank is filled to about 400. Two real-world limits matter: a tank is only filled to about 80 percent to leave room for expansion, so the usable amount is less than the nominal size; and in cold weather the liquid propane vaporizes more slowly, so a small cylinder cannot supply a large burner continuously and may frost up and starve the appliance. This is a planning estimate for sizing and refills; confirm the tank size, the regulator, the vaporization rate for your climate, and the appliance BTU draw with the propane supplier.
anvilfield.com/calculators/propane-tank-runtime-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
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Propane runtime FAQ
Is propane heavier than air?
Yes. Propane has a specific gravity near 1.5, so it is about half again as heavy as air and sinks instead of rising. A leak pools in basements, pits, and low spots where it can reach an ignition source. That is why propane detectors mount low and tanks never go into a confined pit below grade.
What is the difference between propane and natural gas?
Propane is stored as a liquid under pressure and is heavier than air, while natural gas arrives as vapor from a utility main and is lighter than air. Propane carries about 2,516 BTU per cubic foot against roughly 1,030 for natural gas, so it uses smaller orifices, higher appliance pressure, and its own sizing tables.
What is the 80 percent fill rule?
The 80 percent fill rule means a propane tank is filled to no more than 80 percent of its volume by liquid, leaving the top fifth for vapor. Liquid propane expands sharply as it warms, so that headspace gives the expansion somewhere to go. The OPD valve on modern tanks enforces it during filling.
Can you run a natural gas appliance on propane?
No, not without converting it first. Propane carries more than twice the heat per cubic foot and runs at higher pressure, so a natural-gas appliance fed propane through its larger orifices grossly overfires, overheats, and makes carbon monoxide. Convert with the manufacturer's orifices and regulator parts, then set the pressure and check combustion.
Where should a propane gas detector be mounted?
Mount a propane gas detector low, commonly within about 6 to 18 inches of the floor, because propane is heavier than air and settles downward. This is the opposite of a carbon monoxide or natural-gas alarm. Place it near likely leak points and low areas, and follow the detector manufacturer's instructions for height and testing.