How Many Gallons in a 30 lb Propane Tank? Three Ways to Count, One Right Answer

A 30 lb propane tank holds 7.1 gallons of propane at the legal fill. That figure comes from two numbers stamped on the cylinder collar and one physical constant: a water capacity of 71.4 lb, the 42 percent maximum filling density set by 49 CFR 173.304a, and propane's liquid density of 4.24 lb per gallon at 60°F. Forty-two percent of 71.4 lb is 30.0 lb of propane, and 30.0 divided by 4.24 is 7.07 gallons, which Manchester Tank and Worthington Enterprises both publish as 7.1. The bare steel shell would hold 8.6 gallons of anything. You are never sold that last gallon and a half.

Three ways to count, and what each one is really measuring

Ask this question in an RV forum and you will get 6.9, 7.1, and 8.6 gallons in the same thread, each posted with confidence. All three are arithmetically correct. They are answers to three different questions.

| What is being counted | 30 lb cylinder | How the number is produced | |---|---|---| | Liquid propane at the legal fill | 7.1 gal | 42% of 71.4 lb water capacity = 30.0 lb ÷ 4.24 lb/gal | | Total internal volume of the shell | 8.6 gal | 71.4 lb of water ÷ 8.337 lb per gallon | | 80% of the shell (the "80 percent rule") | 6.9 gal | 8.56 gal × 0.80 |

Only the first line answers the question a propane buyer is actually asking, which is how much fuel goes in and how much you pay for. The 8.6-gallon figure is the internal volume of the pressure vessel, about 1,980 cubic inches, and it is exactly what the stamped water capacity describes: the weight of water the empty cylinder would hold. The 6.9-gallon figure is a rule borrowed from a different class of tank, and the next section deals with it.

I have had these three numbers on paper since before it was light, and it is fully light now without my noticing. The arithmetic was never the difficulty. Two of the three answers are correct answers to questions nobody asked.

The two numbers stamped on your collar decide everything

I spent years booking asbestos surveys for a licensed contractor and reading the reports that came back, hundreds of them, without ever setting foot on a site. What that work teaches you is that the document outranks the story. A homeowner would swear the ceiling went up in 1994; the report would find chrysotile in the textured coating and the deeds would say 1974. The stamp wins.

A propane cylinder carries its own report, cold-stamped into the collar:

• WC — water capacity in pounds. On a 30 lb cylinder this is 71.4 or 71.5, depending on the manufacturer.
• TW — tare weight, the empty weight of the cylinder with its valve fitted.
• DOT-4BA240 — the specification and the 240 psig service pressure.
• A month and year — the manufacture date, which starts the requalification clock.

Everything a filler does is derived from those first two marks. Filling density, as 49 CFR 173.304a defines it, is the ratio of the weight of gas in the container to the weight of water the container will hold at 60°F. For commercial propane, whose specific gravity sits between 0.504 and 0.510 in OSHA's maximum permitted filling density table at 29 CFR 1910.110, the limit for an uninsulated cylinder is 42 percent. NFPA 58 uses the same ratio in its own equivalences, pairing a 1,000 lb water capacity with a nominal 420 lb of propane.

That same gravity band is where 4.24 lb per gallon comes from. Water weighs 8.337 lb per gallon at 60°F, and 0.509 of that is 4.244. The density falls out of the same table the filler is working to.

Run 42 percent against the stamped WC and the nominal sizes fall out exactly. 47.6 × 0.42 = 19.99 lb. 71.4 × 0.42 = 29.99 lb. 95.2 × 0.42 = 39.98 lb. The "30 lb" on the label is not marketing. It is the water capacity multiplied by a number written in the Code of Federal Regulations.

The date stamp runs its own clock. A DOT 4BA or 4BW cylinder must be requalified 12 years after manufacture, then every five years under external visual inspection, which is how most are done. Past the date, a refill counter will turn you away.

Why the "80 percent rule" gives you the wrong number for a cylinder

The 80 percent rule is real and widely quoted, and it belongs to bulk ASME tanks. A 500-gallon stationary tank takes 400 gallons of propane, and a technician confirms it by cracking the fixed maximum liquid level gauge until liquid spits out. That method fills by volume.

A DOT cylinder is filled by weight. Paragraph (c) of 173.304a requires the amount of liquefied gas put into a cylinder to be determined by weight, verified on an accurate scale after the fill hose comes off. Nobody watches a bleeder valve on a 30 lb bottle.

Because the two methods use different units, they land in different places. Thirty pounds of propane at 4.24 lb per gallon is 7.07 gallons, and 7.07 into 8.56 is 82.6 percent of the shell by volume at 60°F. So a correctly filled cylinder sits a little above the 80 percent line people expect, and the vapour space is nearer 17 percent than 20. That margin still works because propane expands about 1.5 percent for every 10°F rise. Warm that liquid from 60°F to 100°F on a July afternoon and 7.07 gallons becomes roughly 7.5, or 87.5 percent of the shell, with headspace to spare.

The overfill prevention device does not change this arithmetic. NFPA 58 has required an OPD on cylinders of 4 to 40 lb propane capacity since the 1998 edition, and it is a backstop, not a meter. Its float closes if a fill runs long. The scale is what determines the sale.

Ask a filler for "80 percent" on a 30 lb bottle and you have named a volume nobody at that counter measures.

20 lb vs 30 lb: 51 percent more propane for 17 more pounds

Both are portable DOT cylinders with OPD valves, and the choice is a trade between runtime and what your back and your tank rack will take. Figures below are from Manchester Tank's portable DOT steel LP gas cylinder sheet, whose capacities are nominal at 60°F.

| | 20 lb | 30 lb | 40 lb | |---|---|---|---| | Propane capacity | 4.7 gal | 7.1 gal | 9.5 gal | | Water capacity (WC) | 47.6 lb | 71.4 lb | 95.2 lb | | Tare weight (TW) | 18 lb | 25 lb | 32 lb | | Weight full | 38 lb | 55 lb | 72 lb | | Overall height | 18.0 in | 23.4 in | 29.4 in | | Cylinder diameter | 12.2 in | 12.2 in | 12.2 in | | DOT specification | 4BA240 | 4BA240 | 4BW240 |

The diameter is the same across all three. Only the height changes, which is why a 30 lb cylinder drops straight into a standard RV A-frame rack and will not go under most grill cabinets. Measure the vertical clearance before you buy, not the footprint.

Stepping up from 20 lb to 30 lb buys 51 percent more fuel and costs 17 lb more to lift, every time you lift it. Worthington's steel cylinders are a little lighter than Manchester's — 16.6 lb and 23.3 lb tare respectively, giving 36.6 lb and 53.3 lb full — so read your own stamp rather than a chart.

There is a third option most RV owners never price. Worthington's composite 31 lb cylinder, DOT-SP 12706, has a tare weight of 18.7 lb and holds 7.4 gallons. Full, it weighs about 49.7 lb, roughly four pounds under a full steel 30 lb, with 4 percent more propane in it. It is taller at 28.3 inches and it costs considerably more, and I would still take it over the steel bottle if the rack has the headroom.

One practical wrinkle: exchange cages stock 20 lb cylinders almost exclusively. A 30 lb bottle is refill territory, which works in your favour, since Blue Rhino disclosed a 2008 product content change reducing its exchange fill from 17 lb to 15 lb. That is 3.5 gallons in a cylinder rated for 4.7.

How to find out how much propane is left in your tank

Gauges on portable cylinders are unreliable. Magnetic strips read the temperature of the steel, and the inline pressure dials read vapour pressure, which stays roughly constant until the liquid is nearly gone. The scale is the only honest instrument.

1. Read the TW stamped on the collar. On a 30 lb cylinder expect 23 to 25 lb. Use your cylinder's mark, not a chart value.
2. Weigh the cylinder on a bathroom scale. Valve closed, hose disconnected.
3. Subtract TW from the scale reading. The remainder is the propane in pounds.
4. Divide by 4.24 for gallons remaining.
5. Divide the pounds by 30 for percentage of a full fill.

A worked example: the scale reads 41 lb, the collar is stamped TW 25. That leaves 16 lb of propane, which is 3.8 gallons and 53 percent of a full cylinder. The same method tells you whether a refill was honest. Thirty pounds plus a 25 lb tare should come back off the counter at 55 lb, and if it reads 49, you paid for propane you did not get.

What 7.1 gallons actually runs

Using the EIA's conversion factor of 91,452 Btu per gallon, a full 30 lb cylinder carries about 649,000 Btu. Against real appliance ratings, burning continuously:

• A 30,000 Btu/hr RV furnace: 21.6 hours
• A 12,000 Btu/hr water heater: 54 hours
• A single 9,000 Btu/hr cooktop burner: 72 hours

Continuous burn is a floor. A furnace thermostat cycles, often running about a third of the time in shoulder-season weather, which stretches that 21.6 hours to close to three days. Two 30 lb cylinders on a trailer A-frame hold 14.2 gallons, a fortnight of autumn camping and about a week of hard winter at two gallons a day. The second bottle is there so you find out at the changeover rather than at 3 a.m.

Frequently asked questions

What is the difference between a 20lb and 30lb propane tank?

A 30 lb cylinder holds 7.1 gallons against the 20 lb cylinder's 4.7, about 51 percent more fuel. Both are DOT-4BA240 cylinders of identical 12.2-inch diameter; the 30 lb stands roughly five inches taller and weighs about 55 lb full versus 38 lb. Height, not width, decides whether it fits.

How long will a 30 lb propane tank last?

A full 30 lb cylinder carries about 649,000 Btu, using the EIA figure of 91,452 Btu per gallon. A 30,000 Btu/hr RV furnace burning continuously empties it in 21.6 hours; a 9,000 Btu/hr cooktop burner runs 72 hours. Thermostats cycle, so cold-weather use is usually measured in days.

How much does it cost to fill a 30 pound propane tank?

At the EIA national residential average of $2.674 per gallon for the week ending 30 March 2026, 7.1 gallons is about $19. Counter dispensers charge more, commonly $3.00 to $4.00 per gallon, putting a full refill near $21 to $28. Refills are metered, so a half-empty cylinder costs half.

How many gallons is a 20 lb propane tank?

A 20 lb propane tank holds 4.7 gallons. Its stamped water capacity is 47.6 lb, and 42 percent of that is 19.99 lb of propane, which at 4.24 lb per gallon works out to 4.715 gallons. Exchange-cage cylinders are typically filled to 15 lb, about 3.5 gallons, disclosed in small print on the cage.

How many gallons of propane are in a 40 lb tank?

A 40 lb tank holds 9.4 to 9.5 gallons. Manchester Tank publishes 9.5 gallons on a 95.2 lb water capacity, while the filling-density arithmetic gives 39.98 lb of propane, or 9.43 gallons. It is a DOT-4BW240 cylinder, 29.4 inches tall, roughly 32 lb empty and 72 lb full.

What does a 30 lb propane tank size chart measure?

A size chart lists water capacity in pounds, propane capacity in gallons, tare weight, overall and body height, cylinder and base diameter, DOT specification, and valve type. Water capacity and tare weight are the two that matter, because every fill and every remaining-fuel calculation is built from them.