Propane Tank Calculator — How Long Will It Last, and What It Costs
The boxes start with example numbers so you can see it working — change them to yours and every answer updates as you type. There is nothing to submit.
1. How long will the tank last?
Tank sizes below are usable propane — the 80% fill rule is already taken off, so a “500 gallon” tank is 400.
- Energy in a full one
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- Burns through
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2. What does it cost to run?
No price is assumed here. Propane varies by more than double between a bulk delivery and a bottle swap outside a gas station, and an invented average would be wrong everywhere. Put in what you actually pay.
- A month
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- A year, at this rate
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- Gallons a day
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How long will a 20 lb propane tank last?
A full 20 lb bottle holds about 4.6 gallons, or roughly 430,000 BTU of usable heat. Divide by what you are burning: a 30,000 BTU/hr heater run flat out empties it in about 14 hours, a 10,000 BTU/hr one in about 43. Almost nobody runs flat out, so a heater cycling a third of the time stretches that 14 hours across most of a week. The number that matters is not the size of the bottle, it is the burn rate multiplied by the hours you actually run it.
Why does my propane tank freeze up and stop putting out heat?
Because propane leaves the tank as gas, and the liquid has to boil to replace it. That boiling pulls heat through the wetted wall of the tank, so a small bottle has a small surface to pull it through. Ask for more than it can supply and the tank chills, frost forms on the outside, the pressure sags and the appliance starves — usually at four in the morning in February, which is when you were asking the most of it. A rough ceiling is 35,000 BTU/hr continuous from a 20 lb bottle in mild weather, and less when it is freezing or the bottle is into its last quarter. The fix is never a different regulator. It is a bigger tank, two bottles manifolded together, or asking for less at once.
How much propane does a heater use per hour?
Divide the appliance rating by 91,500 BTU, which is what a gallon of propane holds. A 30,000 BTU/hr heater burns about a third of a gallon an hour. A 12,000 BTU/hr wall heater burns about an eighth. A tankless water heater rated 120,000 BTU/hr burns one and a third gallons an hour, but only while somebody is in the shower — which is why rated input tells you almost nothing until you multiply it by real hours.
What does it cost to run propane for a month?
Gallons burned times what you paid. The trap is the rate: bottle exchange at a shop is commonly two to three times the price per gallon of filling your own tank, and an exchange cage often hands back a bottle filled to 15 lb rather than 20. If you are comparing a delivered tank against swapping bottles, compare cost per gallon, not cost per bottle.
How long does a 100 lb tank or a 500 gallon tank last?
A 100 lb bottle holds about 23.6 gallons, roughly five times a 20 lb. A 500 gallon tank is filled to 80 percent, so about 400 gallons, and that 80 percent is not a safety habit — it is the vapour space liquid propane needs to expand into when it warms. The bigger tank also solves the vaporisation problem above: more wetted wall means far more BTU per hour available before it ever starts to frost.
Sizing the electrical side too? The off-grid power calculator does the array, the battery bank, the panel angle, wire gauge and the inverter.
HomePlot keeps the answers against the actual tank — what you filled, when, and what it cost. Free, in a browser, works with no signal.
