How this is calculated
Three multiplications. Usable fuel is water capacity times the fill limit. Stored energy is usable fuel times energy per gallon or kilogram. Days of supply is stored energy divided by the daily burn.
The appliance side is where people go wrong. A burner is rated by the fuel it swallows per hour, so a 60,000 Btu per hour furnace burns that for each hour the thermostat holds it on and nothing the rest of the day.
Worked through on a 500 gallon tank: 80 percent of 500 is 400 gallons, and at the 91,452 Btu per gallon the Energy Information Administration publishes that is 36.6 million Btu in the yard.
Against it: cooking at 9,000 Btu per hour for ninety minutes, 13,500. Forty gallons of hot water raised 70°F is 40 × 8.34 × 70, or 23,352 Btu, costing 29,190 through an 80 percent tank. A furnace running seven hours, 420,000. Total 462,690 a day, so the tank runs 79 days of that weather, and past 200 once a mild month cuts the furnace to a third of its cold-day hours.
Why the tank stops at 80 percent
Liquid propane swells with temperature far more than water does, and the vapor space above it is the room that swelling needs. Fill a container solid on a cold delivery morning and a warm afternoon has nowhere to put the extra volume except the relief valve.
So the limit is federal law. OSHA 29 CFR 1910.110 Table H-27 writes it as a filling density by weight: 42 percent of water capacity for propane in an uninsulated aboveground container up to 1,200 gallons, 45 percent above that, 46 percent underground.
That is where the cylinder names come from. Manchester Tank gives the barbecue cylinder a water capacity of 47.6 pounds, and 42 percent of 47.6 is 20.0.
Hardware enforces it. The fixed liquid level gauge is a dip tube cut to the 80 percent depth, and the driver stops the moment white liquid sprays from it.
What the dial percentage actually counts
A dial showing 40 percent on a 500 gallon tank means 40 percent of 500, or 200 gallons: half of the 400 you paid for. Temperature shifts it, so compare readings taken in similar weather.
Reading the sizes against each other
The jump that matters is from cylinders to a tank. Five barbecue cylinders make one 100 lb cylinder; twenty make one 120 gallon tank. Cylinders are exchanged by hand, while a tank needs a tanker able to park within 25 meters of it, and access decides more installations than capacity does.
The American ladder: 20 lb for a grill, a pair of 100 lb cylinders for a cabin range and heater, 120 or 250 gallons once a water heater joins in, 500 when a furnace appears, 1,000 for heat plus a generator.
The European ladder is bottles then bulk: 6 kg for camping, 11 to 13 kg as the standard indoor cooking bottle, a 35 kg bottle or a pair for a cooker and water heater, then 1,200 or 2,000 liters above ground, or 3,400 to 4,000 buried.
How fast the vessel can boil in a cold snap
A vessel supplies gas only as fast as the liquid inside can boil, and boiling draws heat through the wetted steel from the air outside. Cold air and a half-empty tank both cut that rate, which is what the last column of the chart measures.
The published rates start at tank sizes. Quality Steel’s chart begins at a 120 gallon tank, 235,008 Btu an hour at 20°F prevailing air with the tank 40 percent full, and climbs to 634,032 for a 500 gallon and 1,088,472 for a 1,000. Calor’s bulk figures land in the same place from the other side: 187 kW for a 2,000 liter tank, which is 638,070 Btu an hour.
Cylinders sit below that floor. A 100 lb cylinder holds about a quarter of the 120 gallon tank’s water capacity, which is why one of them starves a large tankless water heater in January: the cylinder frosts, pressure sags, and the appliance faults on a container that still weighs plenty.
The cure is more wetted steel, not more propane in the same shell. That is why suppliers manifold cylinders in pairs and fours.
Where the vessel is allowed to sit
Setbacks follow water capacity rather than fill level, measured to the nearest important building or a property line that could be built on.
OSHA 29 CFR 1910.110 Table H-23, which tracks NFPA 58: no distance for an aboveground container under 125 gallons water capacity, 10 feet from 125 to 500, 25 feet from 501 to 2,000, 50 feet from 2,001 to 30,000. Underground containers sit 10 feet out, and 251 to 2,000 gallon containers stand 3 feet apart.
British and much of continental practice follows the UKLPG code Calor publishes: 3 meters from buildings, boundaries and fixed ignition sources up to 2,000 liters, 7.5 meters from 3,400 liters up. A firewall cuts those to 1.5 meters and 4 meters respectively. Buried tanks need the ground above clear of structures and deep roots. National rules differ, so the supplier’s siting sheet governs.
Storing the liquid fuels beside it
Propane is the only fuel here that does not age. Sealed under its own pressure with no water and no air in the shell, a full cylinder is as good years later as the day it was filled. Everything else runs a clock: treated diesel and kerosene keep for years sealed and cool, gasoline holds months, pellets are ruined by damp.
So rotate the liquids through something that burns them and let propane be the layer that waits. If a generator is in the plan, size it on the backup power calculator before you size the tank, because its hours swamp everything else in the fuel arithmetic.
Quick answers
How long will a 500 gallon propane tank last?
400 gallons, about 36.6 million Btu: roughly 87 cold-weather days of whole-house heat, or 470 hours of a 5 kW generator.
Why are propane tanks only filled to 80 percent?
The vapor space above the liquid is expansion room for propane that warms. OSHA Table H-27 sets the limit at 42 percent of water capacity by weight for an aboveground container up to 1,200 gallons.
How much propane does a 20 lb cylinder hold?
4.7 US gallons, about 9 kg, near 430,000 Btu or 126 kWh: a month of camp cooking, or an hour and a quarter feeding a 22 kW standby generator at full output.
What is the difference between propane and butane bottles?
Energy per kilogram is nearly the same. What separates them is boiling point: propane works down to −42°C, butane stops near freezing, so a butane bottle outdoors in winter goes quiet while still heavy.
How far does a propane tank have to be from the house?
In the United States, nothing under 125 gallons water capacity, 10 feet to 500, 25 feet to 2,000. In Britain and much of Europe, 3 meters to 2,000 liters, 7.5 from 3,400 up.
Sources
- Fuel energy content. Energy Information Administration, Monthly Energy Review August 2026, Appendix A Table A1 and Table A3: propane 3.841 million Btu a barrel, normal butane 4.353, kerosene 5.670, 15 ppm distillate 5.770, finished motor gasoline 5.047 on the 2026 consumption estimate. Wood at 20 million Btu a cord and natural gas at 1,036 Btu a cubic foot from the same agency’s conversion factors.
- Fill limits and setbacks. OSHA 29 CFR 1910.110, Table H-23 for container location and Table H-27 for maximum filling density, both tracking NFPA 58.
- Vessel specifications. Manchester Tank portable DOT steel LP gas cylinder sheet, 5 lb to 100 lb, capacities nominal at 60°F; Worthington Cylinders 100, 200 and 420 lb sheet.
- Vaporization and off-take. Quality Steel Corporation vaporization chart, Btu per hour with 40 percent liquid in the tank at 20°F and 60°F prevailing air, and its propane properties column for density; Calor specifier’s guide to LPG, Table 2 for maximum off-take and Table 3 for separation distances.
- European bottle sizes. Repsol’s domestic range for the 6 kg and 12.5 kg butane and the 11 kg and 35 kg propane bottles; Calor’s bottle range for the 6 kg and 13 kg propane sizes.
Every figure on this page was re-checked against these sources in September 2026.
Go deeper
Fuel is half a backup plan; the other half is what it has to run. Start with the backup power calculator, then read burn rates off the generator fuel consumption chart. For the wood column, see the firewood BTU chart and wood stove sizing. More under heating and cooling.