Propane tank sizes: how much each one holds

Figures checked Nov. 10, 2026

Big propane tanks are sized in gallons but filled to only about 80% of that, so a 500-gallon tank holds about 400 gallons. Small cylinders are sized by the pounds of propane they hold: a full 20-lb grill cylinder holds about 4.7 gallons. The chart below gives each size we hold figures for.

Tank or cylinder	Propane when full
20-lb cylinder	about 4.7 gallons
30-lb cylinder	about 7 gallons
100-lb cylinder	about 23 gallons
120-gallon tank	about 96 gallons
250-gallon tank	about 200 gallons
325-gallon tank	about 260 gallons
500-gallon tank	about 400 gallons
1,000-gallon tank	about 800 gallons

120 gal tankabout 96 gal usable
250 gal tankabout 200 gal usable
325 gal tankabout 260 gal usable
500 gal tankabout 400 gal usable
1,000 gal tankabout 800 gal usable

Tanks are filled to about 80% of their size, so the usable gallons are less than the size on the tank. Usable figures from propane suppliers' published charts.

How much propane you have

The sums behind this tool are tested against 8 worked examples printed in published sources, and 34 checks in all, including the limits where it gives no number.

Propane tank size chart

The usable gallons for big tanks come from propane suppliers' published size charts. They match the official fill rule: Utah's State Fire Marshal says propane containers are filled to only 80% of their capacity, to allow for expansion.

So the usable figure is the tank's size times 80%. For a size we don't list, multiply its size by 80% to get the gallons in a full tank.

How many gallons are in a propane tank?

For a big tank, take the size on the tank and multiply by 80%. That is a full fill. To see what's in it now, read the gauge: it shows a share of the whole tank, so a full tank reads about 80%, and the calculator turns the reading into gallons.

For a cylinder, it's the pounds of propane divided by the weight of a gallon, about 4.24 pounds by a propane retailer's figure. We found no government figure for the weight of a gallon. Published figures differ slightly, and we use the heavier one, so our gallons lean low.

Standard propane tank sizes: cylinders and tanks

Cylinders are the portable ones, sold by weight: 20 lb for most grills, with 30 lb and 100 lb sizes as well. Their rated weight already allows for the 80% fill, so 80% isn't taken off again.

Tanks are the big ones, sized in gallons and filled to about 80% of that. Our size pages cover the 120 gallon, 250 gallon, 325 gallon, 500 gallon and 1,000 gallon sizes.

Twenty-gallon propane tank or 20-lb cylinder?

If you mean the grill tank, that's the 20-lb cylinder: cylinders are named by pounds of propane, not gallons. If your tank really is sized in gallons, check its data plate and ask your propane supplier.

A county consumer protection column in Pennsylvania also called the grill tank a twenty-gallon tank, adding that it holds about five gallons when full. A 20-lb cylinder holds about 4.7 gallons by suppliers' figures, and exchange cylinders often less. The 20-lb tank page explains why.

Dimensions and empty weights

Tank and cylinder dimensions and empty weights vary from maker to maker, so we don't give them. For a big tank, check the data plate or ask your propane supplier. For a cylinder, the empty weight is stamped on it as TW (tare weight).

Which size suits a home depends on its appliances, and where a tank can sit depends on its size and local code. Ask your propane supplier or local fire marshal, and have tanks and lines installed by a certified propane technician.

Questions people ask

How many gallons does a 500-gallon propane tank hold?

About 400 gallons when full, because big tanks are filled to about 80% of their size.

What size is a standard grill propane tank?

The 20-lb cylinder. It holds about 4.7 gallons when filled to its full rated weight.

Why is a propane tank only filled to 80%?

To leave room for expansion. Utah's State Fire Marshal says propane containers are filled to only 80% of their capacity.

What size propane tank do I need?

That depends on your appliances and how much you use them, so ask your propane supplier. Once you know a size, our calculator shows how long it would run your appliances.

Before you light it

Carbon monoxide (CO) can kill. US Consumer Product Safety Commission (Official): “Fuel-burning heating systems, including furnaces and fireplaces, can produce CO, an odorless, colorless and deadly gas.” Source(opens in a new tab)
Fit CO alarms and test them. US Consumer Product Safety Commission (Official): “CO alarms should be placed on every level of the home and outside sleeping areas.” “Test the alarms every month to make sure they are working.” Source(opens in a new tab)
Have appliances checked every year. US Centers for Disease Control and Prevention (Official): “Have your heating system, water heater and any other gas, oil, or coal burning appliances serviced by a qualified technician every year.” Source(opens in a new tab)
Chimneys and vents too. US Fire Administration (Official): “Have chimneys and vents cleaned and inspected at least once a year.” Source(opens in a new tab)
If a CO alarm sounds (US: call 911): US Centers for Disease Control and Prevention (Official): “If the detector sounds leave your home immediately and call 911.” Source(opens in a new tab)
Know the signs. US Centers for Disease Control and Prevention (Official): “The most common symptoms of CO poisoning are headache, dizziness, weakness, nausea, vomiting, chest pain, and confusion.” Source(opens in a new tab)
Carbon monoxide (CO). CPSC (Official): "Fuel-burning heating systems, including furnaces and fireplaces, can produce CO, an odorless, colorless and deadly gas." "CO alarms should be placed on every level of the home and outside sleeping areas." Source(opens in a new tab)
If you smell gas (US: call 911). Connecticut Public Utilities Regulatory Authority (Official): "If the smell is strong or you are unsure, leave the building and then call." "Go to a phone that is not near the smell and call your propane company right away." "Don't smoke or strike any matches." "Don't flip light switches on or off." "Put out all open flames." "Call your propane company or 911 for help." This is general advice from one state's regulator: follow your own propane supplier's emergency instructions too. Source(opens in a new tab)
Keep propane cylinders outdoors. Utah State Fire Marshal (Official): "Never store a cylinder inside a house, building, or a garage." Source(opens in a new tab)
Outdoor-only heaters, grills and stoves stay outdoors. Mississippi State University Extension (Official), on outdoor-use-only heaters: "Never use them inside, even temporarily." Source(opens in a new tab)

Sources for this page

10 figures, each with its source

Every figure on this page, with its source. Open one to see the exact words on the source page and how it was checked.

Gallons in a full 100 lb cylinder: 23 gallons Other source
From Ferrellgas (propane supplier): How to Choose the Right Propane Tank Size?, July 24 2025, which says: “100-lb: Holds 23 gallons of propane.” Open the source (opens in a new tab)
Checked Nov. 9, 2026. A second check re-opened the source page and matched it on Nov. 9, 2026.
Gallons in a full 20 lb cylinder: 4.7 gallons Other source
From Ferrellgas (propane supplier): How to Choose the Right Propane Tank Size?, July 24 2025, which says: “20-lb: Holds up to 4.7 gallons of propane.” Open the source (opens in a new tab)
Checked Nov. 10, 2026. A second check re-opened the source page and matched it on Nov. 9, 2026. A blind check looked it up again on Nov. 10, 2026 without seeing our figure: the two agreed.
Gallons in a full 30 lb cylinder: 7 gallons Other source
From Ferrellgas (propane supplier): How to Choose the Right Propane Tank Size?, July 24 2025, which says: “30-lb: Holds 7 gallons of propane.” Open the source (opens in a new tab)
Checked Nov. 9, 2026. A second check re-opened the source page and matched it on Nov. 9, 2026.
Weight of a gallon of propane: 4.24 pounds per gallon Other source
From U-Haul (retailer): 20-lb Steel Propane Tank product page (answer in the page's customer Q&A, 2021), which says: “The Propane that should be going into this tank only would weigh about 4.24lbs per gallon of propane.” Open the source (opens in a new tab)
Checked Nov. 10, 2026. A second check re-opened the source page and matched it on Nov. 9, 2026. A blind check looked it up again on Nov. 10, 2026 without seeing our figure: it didn't find a better source, so this figure stands.
Maximum fill of a propane tank: 80% Official
From Utah State Fire Marshal: LP-Gas Basic Filling Procedures, which says: “To allow for expansion, propane containers are to be filled only to 80% of their capacity.” Open the source (opens in a new tab)
Checked Nov. 9, 2026. A second check re-opened the source page and matched it on Nov. 9, 2026. A blind check looked it up again on Nov. 10, 2026 without seeing our figure: the two agreed.
1000-gallon propane tank: usable gallons at 80%: 800 gallons Other source
From Ferrellgas (propane supplier): How to Choose the Right Propane Tank Size?, July 24 2025, which says: “Holds 800 gallons of propane.” Open the source (opens in a new tab)
Checked Nov. 9, 2026. A second check re-opened the source page and matched it on Nov. 9, 2026, and its source label was corrected afterwards.
120-gallon propane tank: usable gallons at 80%: 96 gallons Other source
From Ferrellgas (propane supplier): How to Choose the Right Propane Tank Size?, July 24 2025, which says: “Holds 96 gallons of propane.” Open the source (opens in a new tab)
Checked Nov. 9, 2026. A second check re-opened the source page and matched it on Nov. 9, 2026, and its source label was corrected afterwards.
250-gallon propane tank: usable gallons at 80%: 200 gallons Other source
From Ferrellgas (propane supplier): How to Choose the Right Propane Tank Size?, July 24 2025, which says: “Holds 200 gallons of propane.” Open the source (opens in a new tab)
Checked Nov. 9, 2026. A second check re-opened the source page and matched it on Nov. 9, 2026, and its source label was corrected afterwards.
325-gallon propane tank: usable gallons at 80%: 260 gallons Other source
From Ferrellgas (propane supplier): How to Choose the Right Propane Tank Size?, July 24 2025, which says: “Holds 260 gallons of propane.” Open the source (opens in a new tab)
Checked Nov. 9, 2026. A second check re-opened the source page and matched it on Nov. 9, 2026, and its source label was corrected afterwards.
500-gallon propane tank: usable gallons at 80%: 400 gallons Other source
From Ferrellgas (propane supplier): How to Choose the Right Propane Tank Size?, July 24 2025, which says: “Holds 400 gallons of propane.” Open the source (opens in a new tab)
Checked Nov. 9, 2026. A second check re-opened the source page and matched it on Nov. 9, 2026, and its source label was corrected afterwards.