The formula: usable Btu ÷ Btu per hour
Propane grills, heaters and furnaces state their gas use in Btu per hour, the rate at which they burn fuel. Every tank holds a certain amount of propane, which has a known energy content. Divide one by the other and you have the runtime.
The energy figure comes from the US Energy Information Administration. EIA lists propane at 91,452 Btu per gallon. Its Monthly Energy Review for September 2026 gives the same value as 3.84 million Btu per barrel, and a barrel is 42 US gallons: 3,841,000 ÷ 42 = 91,452 Btu. EIA uses gross heat content, which includes the heat in the water vapor the flame produces. It is the number US energy statistics are built on.
EIA does not publish a per-pound figure, so this guide derives one. NIST's data for liquid propane give a density of 31.631 lb per cubic foot at 60 °F, the temperature that defines the gallon in propane sales. A gallon is 231 cubic inches, so a gallon of liquid propane weighs 4.23 lb, and a pound holds 91,452 ÷ 4.23 = about 21,628 Btu. Propane sold to you is a mixture that can contain some butane, as NIST points out, so treat the last digits as approximate.
If your rating or your fuel is in other units, the energy converter turns Btu into kWh or joules, the weight converter turns pounds into kilograms, and the volume converter turns gallons into liters. The energy units guide explains how Btu, therms and kWh relate.
What each tank size holds
Propane containers go by nominal names. Small cylinders are named in pounds, large tanks in gallons, and neither name is the amount you get.
- 20 lb cylinder. NIST explains that the name is the most propane the cylinder can safely hold. Most cylinders sold through exchange programs are filled and labeled to 15 lb net. A refill at a station can go up to the 20 lb safe fill level.
- 100 lb cylinder. The table assumes the same naming, 100 lb of propane when full. Check the net weight on your delivery slip.
- 500 gallon tank. The gallons are the size of the container, not its contents. The fill limit is explained further down. With the ratio from OSHA's worked example, a 500 gallon above-ground tank holds about 399 gallons when full.
| Container | Propane | Gallons | Pounds | Energy, Btu |
|---|---|---|---|---|
| 1 lb cylinder | if filled to its name | 0.24 | 1 | 21,628 |
| 20 lb cylinder, exchange | filled to 15 lb net | 3.55 | 15 | 324,417 |
| 20 lb cylinder, full | safe fill level | 4.73 | 20 | 432,556 |
| 100 lb cylinder | if filled to its name | 24 | 100 | 2,162,779 |
| 500 gal tank | filled to 79.8% | 399 | 1,687 | 36,489,348 |
Pounds convert to gallons at 4.23 lb per gallon, and every gallon is worth 91,452 Btu. A full 20 lb cylinder therefore holds a little under 5 gallons of liquid propane.
Hours by tank and appliance
The ratings in the column headings are examples, round numbers to show the scale. Appliances of the same kind vary widely, so replace them with the figure on your own appliance's rating plate or in its manual. Each cell is the energy in the container divided by the rating, which is the time at full output without a break.
| Container | Camp stove burner 10,000 Btu/h | Grill, all burners high 30,000 Btu/h | Patio heater 40,000 Btu/h | Generator 60,000 Btu/h | Furnace 80,000 Btu/h |
|---|---|---|---|---|---|
| 1 lb cylinder | 2.2 h | 0.7 h | 0.5 h | 0.4 h | 0.3 h |
| 20 lb cylinder, exchange | 32 h | 11 h | 8.1 h | 5.4 h | 4.1 h |
| 20 lb cylinder, full | 43 h | 14 h | 11 h | 7.2 h | 5.4 h |
| 100 lb cylinder | 216 h | 72 h | 54 h | 36 h | 27 h |
| 500 gal tank | 3,649 h | 1,216 h | 912 h | 608 h | 456 h |
Three things make real life longer than the table. A grill on medium, a heater on its low setting and a generator at part load all burn less than their maximum rating. A furnace or water heater cycles on and off, so a figure of several hundred burner hours lasts many days. And a cylinder is rarely completely full when you start. The table is the floor, not the forecast: at full output you will not get more than this.
Find the right number for your appliance
The runtime is only as good as the Btu per hour you divide by. A few places to look:
- Rating plate or label. Look for the Btu per hour figure on the plate or label. If it lists both an input and an output figure, use the input: that is what leaves the tank.
- Several burners. A grill or range may list each burner separately. Add the burners you actually use; a side burner you never light does not count.
- Settings. If a heater lists ratings for low and high, run the numbers for the setting you use most.
- Generators. A generator's watts are its electrical output, not what it burns. Look in the manual for fuel use per hour, which may be given in pounds or gallons at a stated load.
If the manual lists pounds per hour, you do not need Btu at all: divide the pounds in the tank by the pounds per hour. Example: a manual that lists 2.2 lb per hour at half load would empty a 100 lb cylinder in 100 ÷ 2.2 = 45 hours. In Btu that load is 2.2 × 21,628 = 47,581 Btu per hour. If the figure is in gallons per hour, divide the tank's gallons instead.
The BTU calculator works the other way round: it estimates how many Btu per hour a room needs. That is the right tool when you are choosing an indoor heater or air conditioner. This guide starts from the appliance you already have and asks how long the fuel lasts.
How much is left: weigh the cylinder
NIST notes that propane in 20 lb cylinders must legally be sold by weight, so weight is also the honest way to see what is left. It also notes that every cylinder carries its tare weight, stamped or stenciled after the letters TW. The tare covers the empty cylinder, its valve and other permanent attachments.
Example: NIST's survey report uses a 20 lb cylinder with a tare of 18 lb. If yours shows 27 lb on a bathroom scale, 9 lb of propane is left: 9 × 21,628 = 194,650 Btu. On a 30,000 Btu per hour grill with every burner on high, that is 6.5 hours. If you cook at about half that rate, it is closer to 13 hours.
The same check is worth doing before you hand in a cylinder for exchange. In the NIST survey of returned cylinders, about a quarter of customers brought back more than 1 lb of propane, and 9.1 percent of cylinders still held more than 5 lb. That is fuel paid for and given back.
Big tanks and home heating: use your delivery history
For a house, the table of burner hours is less useful than your own records. A furnace runs more on cold days and less on mild ones, and the same tank may also feed a water heater, a range or a dryer. Your delivery history already contains all of that.
The tool that ties gallons to weather is the heating degree day. EIA defines it with a base of 65 °F: a day with a mean temperature of 20 °F counts 65 − 20 = 45 heating degree days. The mean is the day's high plus low, divided by two. Add up the degree days between two deliveries and divide the gallons delivered by that sum.
Example with made-up numbers: 180 gallons refilled the tank after a stretch with 1,200 heating degree days, so the house uses 0.15 gallons per degree day. The gauge now reads 35 percent of a 500 gallon tank, 175 gallons.
| Weather | Degree days per day | Gallons per day | Days left |
|---|---|---|---|
| Mean 45 °F | 20 | 3.0 | 58 |
| Mean 20 °F | 45 | 6.8 | 26 |
The same tank lasts more than twice as long in mild weather as in a cold spell, which is why a fixed rule of thumb in days per tank fails. Order before the gauge gets low: a delivery takes time, and a cold week can use up a margin quickly. The figure includes hot water and cooking, because those gallons came from the same tank. If you heat with utility gas instead, the guide to reading your gas bill shows where the therms on the bill come from; 1 therm is 100,000 Btu, per EIA.
Cold weather: pressure drops, the gallon gets heavier
Propane sits in the tank as a liquid under its own vapor pressure, and the appliance burns the vapor. How much pressure the tank holds depends on the temperature of the liquid. NIST's property data for propane show how quickly it falls:
| Liquid temperature | Vapor pressure, psia | Above the atmosphere, psi | Weight per gallon, lb |
|---|---|---|---|
| -40 °F | 16.1 | 1.4 | 4.83 |
| -20 °F | 25.4 | 10.7 | 4.72 |
| 0 °F | 38.4 | 23.7 | 4.60 |
| 20 °F | 55.8 | 41.1 | 4.49 |
| 40 °F | 78.6 | 63.9 | 4.36 |
| 60 °F | 107.7 | 93.0 | 4.23 |
| 80 °F | 144.0 | 129.3 | 4.09 |
| 100 °F | 188.6 | 173.9 | 3.93 |
Psia is absolute pressure; the atmosphere itself is 14.7 psia, so the third column is roughly what a gauge would show. At 60 °F there is about 93 psi above the atmosphere to push gas to the appliance. At 0 °F there are about 24, and propane boils at about -44 °F, where the pressure is no higher than the air outside. Less pressure means less push to move gas to the burner, even when there is plenty of fuel left. The manual of your appliance states the conditions it is designed for.
The energy per pound does not change with temperature, so weight stays the reliable measure. Volume does change: the last column shows that a cold gallon holds more propane than a warm one.
Why a tank is never filled to the top
Liquid propane expands as it warms. From NIST's densities, the same pounds of propane take up 6.7 percent more room at 80 °F than at 40 °F. A tank filled to the brim on a cold morning would have no space left for that expansion on a warm afternoon.
Federal rules therefore cap the fill. OSHA's standard for liquefied petroleum gases sets a maximum filling density, the weight of gas compared with the weight of water the container could hold. It works through an example: a 100 gallon above-ground container of propane may hold at most 79.8 gallons. The same filling density applies to above-ground containers up to 1,200 gallons, so this guide uses 79.8 percent for the 500 gallon tank. Small cylinders reach the same goal by weight, through the 20 lb safe fill level.
For runtime this means one thing: never calculate with the container's full size. Use the pounds or gallons actually inside.
Exchange or refill?
For the 20 lb size the choice changes how long a cylinder lasts. An exchange cylinder filled to 15 lb holds 324,417 Btu; a refill to the 20 lb safe fill level holds 432,556 Btu, a third more. Compare prices per pound, not per cylinder: divide the price by 15 for an exchange and by the pounds actually pumped for a refill. NIST's survey found that refill stations price in several ways (by weight, by volume, or a flat fee), so ask how much went in. Whichever you choose, weigh the old cylinder before you hand it back.
Safety in brief
- Check for leaks. CPSC advises inspecting grill hoses for cracks, brittleness, holes and leaks, and checking connections by opening the gas supply valve fully and brushing on a soapy solution. Bubbles mean a leak. NFPA recommends this check before the first use each year.
- If you smell gas at the grill. NFPA says to get away from the grill immediately and call the fire department. If the flame goes out, turn the grill and gas off and wait at least 15 minutes before relighting.
- Outdoors only. CPSC says to use grills outside in a well-ventilated area. Run a generator outdoors only, at least 20 feet from homes and buildings, with the exhaust pointed away from windows, doors and vents. Never run one in a home, garage, basement, crawlspace or shed, even with doors or windows open.
- Alarms and gas leaks at home. CPSC recommends working carbon monoxide and smoke alarms on every level. If you smell or hear gas leaking in your home, leave immediately.
Frequently asked questions
How long does a 20 lb propane tank last on a grill?
A full 20 lb cylinder holds about 432,556 Btu, from EIA's 91,452 Btu per gallon and NIST's density of liquid propane. A grill rated at 30,000 Btu per hour with every burner on high empties it in about 14 hours; an exchange cylinder filled to 15 lb lasts about 11 hours. At lower settings the grill burns less, so real cooking time is longer.
How many Btu are in a pound of propane?
EIA lists 91,452 Btu per gallon. A gallon of liquid propane weighs about 4.23 lb at 60 °F, from NIST's density data, so a pound holds about 21,628 Btu. EIA's figure is the gross heat content, the number US energy statistics use.
Why does my 20 lb cylinder only have 15 lb in it?
NIST explains that a 20 lb cylinder is named for the most propane it can safely hold, but most cylinders sold through exchange programs are filled and labeled to 15 lb net. A refill at a station can bring it up to the 20 lb safe fill level. The label on an exchange cylinder states the net weight.
How do I check how much propane is left?
Weigh the cylinder and subtract the tare weight stamped on its collar after the letters TW. NIST notes that the stamped tare covers the empty cylinder, valve and permanent attachments. A reading of 27 lb on a cylinder stamped TW 18 means about 9 lb of propane is left.
Does cold weather make propane run out faster?
The energy in a pound of propane does not change with the weather, but the pressure in the tank does. NIST data put the vapor pressure of propane at 108 psia at 60 °F and only 38 psia at 0 °F; propane boils at about -44 °F. A heater also runs longer on a cold day, so the tank empties sooner in calendar time.
Sources
- U.S. Energy Information Administration: British thermal units (Btu), sample Btu conversion factors (propane 1 gallon = 91,452 Btu; natural gas 1 therm = 100,000 Btu)
- U.S. Energy Information Administration: Monthly Energy Review, September 2026, Appendix A, Table A1 (propane 3.841 million Btu per barrel; gross heat content used in all MER Btu calculations)
- U.S. Energy Information Administration: Energy units and calculators explained (1 barrel of petroleum products = 42 U.S. gallons)
- U.S. Energy Information Administration: Degree days (base temperature 65 °F; heating degree days = 65 °F minus the day's mean temperature)
- NIST Chemistry WebBook: thermophysical properties of propane, saturation properties by temperature (liquid density and vapor pressure, −40 to 100 °F)
- NIST Chemistry WebBook: propane, phase change data (normal boiling point 231.1 K)
- NIST Office of Weights and Measures: Liquified Petroleum Gas 20 Pound Cylinders, Measurement Matters, July 2021 (20 lb safe fill, 15 lb net exchange fill, stamped tare weight TW, the gallon as 231 cubic inches at 60 °F)
- NIST Special Publication 2200-01: 2022 NCWM-NIST National Survey on 20 lb LPG (Propane) Cylinders, October 2022 (safe fill level, 18 lb tare example, propane left in returned cylinders)
- Occupational Safety and Health Administration: 29 CFR 1910.110, Storage and handling of liquefied petroleum gases (filling density; (b)(19)(v) worked example of 79.8 gallons maximum in a 100-gallon above-ground container)
- U.S. Consumer Product Safety Commission: Fire It Up Safely, CPSC Recommends Safety Check Before Grilling This Summer, May 25, 2011 (hose inspection, soapy-solution leak check, use grills outside only)
- U.S. Consumer Product Safety Commission: CPSC Warns of Generator, Carbon Monoxide and Fire Hazards Ahead of Hurricane Season, May 27, 2026 (generators outdoors only, at least 20 feet from buildings; CO alarms; leave home if you smell or hear gas)
- National Fire Protection Association: Grilling safety tip sheet, copy published by the North Carolina Department of Agriculture Standards Division (leak check each year, what to do if you smell gas, wait 15 minutes after the flame goes out)