The propane in a 20 lb tank is about 20 pounds, or roughly 4.7 gallons. That equals about 430,000 BTU, or about 11 to 14 hours on a grill burning 30,000 to 40,000 BTU per hour. A full cylinder weighs about 37 lb on a scale, and exchange tanks are often filled to only 15 to 17 lb.
Propane in a 20 lb tank: pounds, gallons and BTU
A 20 lb cylinder is rated to hold 20 pounds of liquid propane. Propane weighs about 4.24 lb per gallon at 60°F, so 20 lb works out to about 4.7 gallons (20 ÷ 4.24).
The tank is never filled to the brim. Liquid propane expands as it warms, so cylinders are filled to no more than about 80% of their internal volume, leaving vapor space. The 20 lb rating already reflects that limit.
| Measure | Full 20 lb tank | How it is figured |
|---|---|---|
| Propane by weight | About 20 lb | Rated fill at the 80% limit |
| Propane by volume | About 4.7 gal | 20 lb ÷ 4.24 lb/gal |
| Energy per pound (estimate) | About 21,500 BTU/lb | 91,500 BTU/gal ÷ 4.24 lb/gal, rounded |
| Energy, full tank (estimate) | About 430,000 BTU | 20 lb × 21,500 BTU/lb |
| Typical exchange fill | 15 to 17 lb (about 3.5 to 4 gal) | 15 to 17 lb ÷ 4.24 lb/gal |
| Total weight, full | About 37 lb | About 17 lb tare + 20 lb propane |
The 91,500 BTU per gallon figure is a standard heating value for propane, and the energy rows are estimates built from it. Tare weight, exchange fill amounts and appliance burn rates vary by model; check the label or manual.
How much does a full 20 lb propane tank weigh?
A full 20 lb tank weighs about 37 lb. That is the empty cylinder (tare weight) plus 20 lb of propane.
The tare weight is stamped on the collar, the metal ring around the valve, and is marked TW. Use the number on your own tank, because each cylinder has its own tare.
An empty tank weighs its TW. A full one weighs TW plus about 20 lb. Everything in between is simple subtraction, covered next.
How to check how much propane is left
Weighing is the most accurate method for a standard grill cylinder.
- Close the tank valve and disconnect the regulator or hose.
- Read the TW stamped on the collar (for example, TW 17).
- Set the tank on a bathroom scale and note the total weight.
- Subtract TW from the scale reading. The result is pounds of propane left.
Worked example: TW is 17 lb and the scale reads 30 lb. 30 − 17 = 13 lb left, which is 65% of 20 lb. That is about 3.1 gallons (13 ÷ 4.24) and 279,500 BTU (13 × 21,500 BTU per pound). At 35,000 BTU/hr, that is about 8 hours of burn time (279,500 ÷ 35,000).
Decision rule: refill or swap when 5 lb or less of propane remains, meaning the scale reads TW plus 5 lb (22 lb for a TW of 17). Five pounds is 107,500 BTU (5 × 21,500), or about 3.6 hours at 30,000 BTU/hr, which will not finish a long cookout. The 13 lb tank in the example sits 8 lb above that line, so it covers the cookout, but plan a swap soon after.
Quicker, less precise checks
- Inline gauge: installs between the tank and regulator and shows a rough level. Fine for a glance, less exact than a scale.
- Warm-water test: pour warm tap water down the side of the tank, then run your hand along it. The metal feels cool where liquid propane sits. Use warm water only, never boiling. This is a rough rule of thumb, not a measurement.
A common mistake is weighing with the hose still attached or comparing against another tank’s TW.
How long does a 20 lb propane tank last?
Divide the tank’s energy by the appliance’s burn rate: 430,000 BTU ÷ BTU per hour = hours at full output. The hours below are estimates that assume the burner runs at the stated rating the whole time. Find the actual rating on your appliance’s label or manual.
| Appliance | Burn rate (BTU/hr) | Full 20 lb tank (430,000 BTU) | 15 lb exchange fill (322,500 BTU) |
|---|---|---|---|
| Gas grill, all burners high | 40,000 | About 10.8 hours | About 8.1 hours |
| Smaller grill | 30,000 | About 14.3 hours | About 10.8 hours |
| Patio heater | 40,000 | About 10.8 hours | About 8.1 hours |
| Fire pit | 50,000 | About 8.6 hours | About 6.5 hours |
| Camp stove burner | 8,000 | About 53.8 hours | About 40.3 hours |
The burn rates are example values. The exchange column uses 15 lb × 21,500 BTU per pound = 322,500 BTU. Real cooking rarely holds a grill at maximum, so grills often last longer than the table shows.
What shortens runtime
- Cold weather: propane vaporizes more slowly in the cold, which can reduce output when the tank is low, so heaters and grills are often run harder.
- Wind: it pulls heat off the cooking surface or heater, so you burn more fuel for the same result.
- High settings: doubling the burn rate halves the hours.
For runtime math across other tank sizes, see the guide on how long propane lasts.
Refill vs. exchange: how much propane do you really get?
A refill is filled by weight, up to the 20 lb rating and the 80% volume limit. An exchange swaps your tank for a prefilled one, and those are often filled to only about 15 to 17 lb. The packaging may say so, but the tank looks and feels like a full one.
Compare cost per pound, not per tank. Example only, not market prices: $20 for a 15 lb exchange is about $1.33 per pound, while $20 for a 20 lb refill is $1.00 per pound. Prices vary by region and retailer.
- Choose refill if you want the full 20 lb, your tank is in good condition and a refill station is nearby. It loses on convenience, since you need a station with hours that suit you.
- Choose exchange if your tank is old, damaged or out of date, or you want a quick swap anywhere. It loses on fill amount and often on cost per pound.
Never overfill. Cylinders need about 20% vapor space, and 20 lb tanks have an overfill prevention device (OPD) valve, required on cylinders from 4 to 40 lb. A refill station should fill by scale weight, not by guess. Shouldn’t top off a tank yourself: leave it to the station.
Storing and handling a 20 lb tank safely
Propane is flammable and heavier than air, so leaks pool low. A few rules cover most risks:
- Keep the tank upright, secured and outdoors in a ventilated spot, away from ignition sources and heat.
- Never leave it in a hot car. Transport it upright and secured, and drive straight home.
- Check connections for leaks with soapy water. Bubbles mean a leak. Never use a flame to test.
- Use grills, patio heaters, fire pits and camp stoves outdoors only. Burning propane indoors can produce dangerous carbon monoxide.
- Don’t use a tank that is dented, badly rusted, leaking or past its requalification date.
Indoor and garage storage rules vary by local and fire code, so read the guide on whether you can store propane tanks inside before choosing a spot.
Propane is odorized to smell like rotten eggs. If you smell gas, move away from ignition sources, shut the valve if it is safe to do so, and call your gas supplier or 911.
Related reading: What Size Propane Tank for a Generac Generator? · Is It Safe to Store a Propane Tank in a Garage?
Frequently Asked Questions
Can a refill station fill my exchange tank to a full 20 lb?
Yes, if the tank is in good condition and within its requalification date. The station fills by scale weight up to the 20 lb rating, which gives you more propane than a 15 to 17 lb exchange fill.
How often does a 20 lb tank need to be recertified?
A new DOT cylinder is typically good for 12 years from the manufacture date stamped on the collar. After that it must be requalified, and the requalification lasts 5 years. Exchange providers handle this, and refill stations may refuse expired tanks.
Can I lay a 20 lb tank on its side to use it?
No. Grills and heaters are designed to draw vapor from an upright tank. A tank on its side can feed liquid propane into the regulator and hose, which can cause flare-ups. Keep it upright and secured.
What does the scale read when a tank is half full?
Half full is 10 lb of propane, so the scale reads TW plus 10 lb. With a TW of 17, that is 27 lb. This is about 215,000 BTU (10 × 21,500), or roughly 6 hours at 35,000 BTU/hr.
Can I use one 20 lb tank on a patio heater or fire pit?
Yes, if the manual lists a 20 lb tank and you use the hose and regulator it specifies. At 40,000 BTU/hr, a full tank lasts about 10.8 hours; at 50,000 BTU/hr, about 8.6 hours.
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