Propane tank runtime calculator
Every propane appliance has a Btu-per-hour rating on its label. Divide the energy in the tank by that rating — adjusted for how much of the time it is actually burning — and you have the hours.
Enter your measurements
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
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
Quick answers for common sizes
| Example | Answer |
|---|---|
| How long a 20 lb propane tank lasts for heating | 24 hours at 18,000 Btu/h |
| 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 |
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 |
Worked examples: how long a 20 lb propane tank lasts for heating, how long a 20 lb propane tank lasts on a grill and how long a 100 lb propane tank lasts. Every size is in the propane tank size chart.
How it is worked out
gallons = pounds of propane ÷ 4.23 Btu in the tank = gallons × 91,452 hours = Btu in the tank ÷ (appliance Btu per hour × share of time on)
The EIA puts propane’s heat content at 91,452 Btu per gallon. Tanks are sold by the weight of propane they hold, so the calculator converts pounds to gallons using NIST’s density for liquid propane at 60 °F — about 4.23 lb per gallon. A full 20 lb cylinder therefore holds about 4.7 gallons and 432,000 Btu.
| Tank | Gallons | Btu | Hours at 10,000 Btu/h | Hours at 30,000 Btu/h |
|---|---|---|---|---|
| 1 lb | 0.24 | 21,620 | 2.2 | 0.7 |
| 20 lb | 4.73 | 432,397 | 43.2 | 14.4 |
| 30 lb | 7.09 | 648,596 | 64.9 | 21.6 |
| 40 lb | 9.46 | 864,794 | 86.5 | 28.8 |
| 100 lb | 23.64 | 2,161,986 | 216.2 | 72.1 |
Why your tank empties sooner (or later)
- The rating is the maximum. Heaters on low, grills on medium and stoves on a simmer burn less. Set the share of time under Share of the time at that rating.
- Thermostats cycle. A furnace or heater on a thermostat runs only part of the hour.
- Tanks are rarely completely full or completely empty. Weigh the tank and subtract the stamped tare weight (TW) for the real amount left.
- Cold slows vaporization. In freezing weather a small tank can’t always deliver a big burner’s full rating.
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?
It holds about 432,000 Btu, so about 24 hours on an 18,000 Btu/h heater, 48 hours at 9,000, or 14.4 hours at 30,000 — at full output. See how long a 20 lb propane tank lasts for heating.
How many gallons are in a 20 lb propane tank?
About 4.7 gallons when full: 20 lb ÷ 4.23 lb per gallon (NIST’s density for liquid propane at 60 °F).
How many Btu are in a gallon of propane?
91,452 Btu, according to the EIA.
How long does a 100 lb propane tank last?
About 2.16 million Btu when full — 72 hours at 30,000 Btu/h, about 43 hours at 50,000. See how long a 100 lb propane tank lasts.
Related guides
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.