How long will a 20 lb propane tank last for heating?
A full 20 lb tank holds about 432,397 Btu, so an 18,000 Btu-per-hour heater on high runs about 24 hours — longer on a lower setting.
How to work it out
- 20 lb ÷ 4.23 lb per gallon = 4.73 gal × 91,452 Btu = 432,397 Btu
- 432,397 Btu ÷ (18,000 Btu/h) = 24 hours
Portable and tank-top propane heaters are rated from a few thousand to tens of thousands of Btu per hour, and many have high and low settings. Check the label for yours; this page uses 18,000 Btu/h on high.
Adjust for your project
More options: Full · Duty
- In hours and minutes
- 24 h 1 min
- In days (round the clock)
- 1 day
- Propane in gallons
- 4.73 gal
- Energy in the tank
- 432,397 Btu
- Propane used per hour
- 0.83 lb
The same appliance on other tank sizes
| Tank | Runtime | Propane in gallons | Energy in the tank |
|---|---|---|---|
| 1 lb | 1.2 hours | 0.24 gal | 21,620 Btu |
| 20 lb | 24 hours | 4.73 gal | 432,397 Btu |
| 30 lb | 36 hours | 7.09 gal | 648,596 Btu |
| 40 lb | 48 hours | 9.46 gal | 864,794 Btu |
| 100 lb | 120.1 hours | 23.64 gal | 2,161,986 Btu |
What to know about heating with a 20 lb tank
- Low setting stretches it. At 9,000 Btu/h the same tank lasts about 48 hours; a heater that cycles on a thermostat lasts longer still.
- Big heaters drain it fast. A 30,000 Btu/h heater on high empties it in about 14.4 hours; at 60,000 Btu/h in about 7.2 hours, and a small tank may struggle to feed that much in cold weather.
- Indoors only if rated, and ventilated. Only heaters rated for indoor use belong inside, with the ventilation their manual requires and a CO alarm nearby. Outdoor and tank-top heaters stay outdoors.
- Compare electric. The same 432,000 Btu is about 127 kWh — more than a hundred times what a 1,000 Wh power station holds, which is why batteries make poor heaters.
What an electric heater costs instead is on the cost to run a space heater, and how fast it drains a battery on how long a power station runs a space heater. Bigger tanks: how long a 100 lb propane tank lasts.
Different size? Use the full Propane Tank Runtime Calculator.
What you’ll need
- Propane tank gaugeBudget price
Shows roughly how much is left so you are not guessing halfway through a cold night or a long outage.
See options on Amazon (opens in a new tab) - Battery-operated carbon monoxide alarmBudget price
CPSC advises battery or battery-backup CO alarms whenever a generator is in use. Look for UL 2034 certification and test monthly.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How long will a 20 lb propane tank last at 9,000 Btu?
About 48 hours (2 days).
How long will a 20 lb propane tank last at 30,000 Btu?
About 14.4 hours (14 h 25 min).
How long will a 20 lb propane tank last at 50,000 Btu?
About 8.6 hours (8 h 39 min) — if the cylinder can keep up in the cold.
Other common sizes
- How long a 20 lb propane tank lasts on a grill — 14.4 hours at 30,000 Btu/h
- How long a 100 lb propane tank lasts — 72 hours at 30,000 Btu/h
Sources
- U.S. Energy Information Administration — British thermal units (Btu) (opens in a new tab) — Heat content: gasoline 120,214 Btu/gal, diesel 137,381 Btu/gal, propane 91,452 Btu/gal; 1 kWh = 3,412 Btu.
- NIST Chemistry WebBook — Thermophysical properties of propane (saturated liquid) (opens in a new tab) — Liquid propane is about 31.6 lb/ft³ at 60 °F, which is 4.23 lb per US gallon (4.09 at 80 °F).
- U.S. Consumer Product Safety Commission — Portable Generator Hazards (Safety Alert 5123, 2021) (opens in a new tab) — Never run a generator indoors or in a garage, even with doors open; place it outside far from doors, windows and vents; CO alarms; never “backfeed” by plugging into a wall outlet.