Fuel Tank Runtime Calculator
Free tool
Whether you're heating a van, a garage or a whole house, the question is the same: how many hours per fill, how many refills a season, and what that costs.
- Propane, oil, diesel, kerosene
- Accounts for cycling
- Season cost included
A 20 lb BBQ cylinder holds about 4.7 usable gallons. Tanks are only filled to 80% for safety.
A thermostatic heater cycles on and off. At 100% it never stops — only true in the coldest conditions or with no thermostat.
30,000 BTU/hr at 70% duty · 4.7 usable gallons of propane
Planning a winter's worth of fuel?
What this calculator does
It works out how long a tank of propane, heating oil, diesel or kerosene lasts on your heater, how many refills a season takes, and what that fuel costs you.
The formula
- Propane — 91,502 BTU per gallon
- Heating oil — 138,500 BTU per gallon
- Diesel — 137,000 · kerosene — 135,000
Why a 20 lb tank is only 4.7 gallons
Propane expands as it warms, so cylinders are filled to 80% capacity by law. A "20 lb" tank holds about 4.7 usable gallons, not the 5.9 its volume suggests. Every figure here uses the usable amount.
Duty cycle matters more than tank size
Duty cycle is the share of time the burner actually fires. A thermostatic heater reaches temperature and shuts off, so it may only run 60–70% of the time. Any calculation assuming 100% will badly understate how long your fuel lasts — often by a third.
Planning a season
- A typical home burns 500–800 gallons of heating oil per winter
- A 275-gallon basement tank therefore needs 2–3 fills
- Buying in late summer usually beats mid-winter pricing
Safety first: only run a heater rated for indoor use indoors, always ventilate, and fit a carbon monoxide alarm in any room where fuel is burned.
Same heater, same duty cycle — only the tank changes. Your tank is highlighted.
| Tank | Usable fuel | Runtime | Cost per fill |
|---|
Propane and similar tanks are filled to 80% capacity by law to allow for expansion, so a "20 lb" cylinder gives about 4.7 usable gallons. Season figures assume a six-month heating season.
Get your runtime plus the winter fuel planner
We'll email your numbers along with the planner that works out how much fuel to buy before prices climb.
- Your runtime, refills and season cost
- When fuel is cheapest to buy
- Safe storage and cold-weather handling
How long does a 20 lb propane tank last on a heater?
A 20 lb cylinder holds about 4.7 usable gallons, or roughly 430,000 BTU. On a 30,000 BTU/hr heater running flat out that is about 14 hours; at a realistic 70% duty cycle, closer to 20 hours. A small 9,000 BTU heater on the same tank runs for two to three days.
Why is a 20 lb tank only 4.7 gallons?
Propane expands as it warms, so tanks are filled to 80% capacity by law. A "20 lb" tank holds about 4.7 usable gallons rather than the 5.9 its volume suggests. Every figure in this calculator uses the usable amount.
What is a duty cycle and why does it matter?
It is the share of time the burner is actually firing. A thermostatic heater reaches temperature and shuts off, so it might only fire 60–70% of the time. Runtime calculations that assume 100% will always understate how long your fuel lasts.
How much heating oil does a house use in winter?
A typical home burns 500–800 gallons across a heating season, though a cold climate and a poorly insulated house can push past 1,000. A standard 275 gallon basement tank therefore needs filling two to three times a winter.
Can I use a propane heater indoors?
Only one rated for indoor use, with an oxygen depletion sensor, and only with ventilation. Never run an outdoor or construction heater inside. Fit a carbon monoxide alarm in any room where fuel is burned — this is not optional.
