Propane Tank Sizes
Propane tanks are sized two different ways: small portable cylinders are rated by the weight of propane they hold, while larger stationary tanks are rated by water-capacity gallons. The tables below compare common sizes with approximate dimensions and the usable propane each holds.
Approximate propane tank planning estimates based on common nominal sizes. Actual dimensions and capacity vary by manufacturer and model, and usable propane depends on temperature, the 80% fill rule, and local fill practices.
Propane pounds-to-gallons (portable cylinders)
Small portable cylinders are rated by the weight of propane they hold, not gallons. Propane weighs about 4.24 pounds per US gallon at around 60°F, so to convert pounds of propane to gallons, divide the weight by 4.24. Because this rating already refers to the propane in the cylinder, the 80% fill rule is not applied again on top of it.
| Tank size | Approx. propane gallons | Common use | Notes |
|---|---|---|---|
| 15 lb | 3.5 gal | Smaller exchange cylinders, some grills | Often a 15-lb fill of a 20-lb cylinder; weight varies by source |
| 20 lb | 4.7 gal | BBQ grills, patio heaters | Most common exchange cylinder; ≈ 18 in tall × 12 in dia. |
| 30 lb | 7.1 gal | RVs, larger grills | ≈ 24 in tall × 12.5 in dia. |
| 40 lb | 9.4 gal | RVs, food trucks, livestock heaters | ≈ 29 in tall × 12.3 in dia. (12.3–12.5 in by manufacturer) |
| 100 lb | 23.6 gal | Fireplaces, dryers, small appliances | ≈ 48 in tall × 14.7 in external dia. (≈ 14 in body diameter) |
Gallons come from dividing the rated propane weight by 4.24 lb/gal, so a "20 lb" grill cylinder holds about 4.7 gallons and a 100-lb cylinder about 23.6 gallons.
The 420 lb / 120-gallon tank
One common tank sits on the boundary between the two systems and is named both ways, which produces two slightly different figures:
- By its 120-gallon water capacity and the 80% fill rule: about 96 gallons usable (120 × 0.8).
- By its 420 lb propane rating divided by 4.24 lb/gal: about 99 gallons.
The small difference is not a contradiction: these are two nominal ratings of the same physical tank. The 80% rule applies to the water-capacity figure, while the 420 lb figure already describes a quantity of propane, so it is not reduced by 80% a second time. Naming varies by supplier and manufacturer, so the same tank may be advertised as a "420 lb" cylinder or a "120-gallon" tank.
Residential propane tank sizes and dimensions
Stationary residential tanks are rated by water capacity, so their usable propane is about 80% of that capacity. Dimensions below are typical and vary by manufacturer, model, and whether the tank is mounted vertically or horizontally.
| Nominal tank size | Usable propane (≈ 80%) | Typical length / height | Typical diameter | Common use | Notes |
|---|---|---|---|---|---|
| 120 gal | 96 gal (≈ 407 lb) | 4 ft 6 in (vertical) / ≈ 5 ft 8 in (horizontal) | 24–30 in | Hot water plus 1–2 appliances | Also sold as a 420 lb cylinder; vertical and horizontal styles differ |
| 250 gal | 200 gal (≈ 848 lb) | ≈ 7 ft 10 in | 30 in | Home heating in milder climates | — |
| 330 gal | 264 gal (≈ 1,119 lb) | 9 ft 7 in – 10 ft | 30–31 in | Whole-home heating, mid-size homes | Less standardized; dimensions vary by maker |
| 500 gal | 400 gal (≈ 1,696 lb) | ≈ 9 ft 11 in | 37.5 in | Whole-home heating, larger homes | Most common residential heating size |
| 1000 gal | 800 gal (≈ 3,392 lb) | 15 ft 10 in – 16 ft 2 in | 41 in | Large homes, farms, light commercial | Dimensions vary by manufacturer |
Usable propane is shown after the 80% fill rule, so a 500-gallon tank holds about 400 gallons of propane when "full" and a 1000-gallon tank about 800.
The 80% fill rule for stationary tanks
Stationary propane tanks are not filled all the way with liquid propane. Propane expands as temperature rises, so tanks are normally filled to about 80% of their water capacity to leave vapor space for expansion. That is why a 500-gallon tank holds about 400 gallons of usable propane when “full,” and a 1000-gallon tank holds about 800 gallons.
For a quick estimate, multiply the tank’s water capacity by 0.8. For example, a 250-gallon tank holds about 250 × 0.8 = 200 gallons of usable propane. The 80% fill limit is part of NFPA 58, the LP-Gas Code, and helps prevent dangerous overfilling as temperatures change.
Choosing a tank size
Smaller cylinders from 20 to 100 lb are typically used for grills, patio heaters, fireplaces, RVs, and single appliances. A 120-gallon tank can cover hot water plus a few propane appliances. The 250 and 330-gallon sizes are often used for smaller homes, milder climates, or moderate heating demand. A 500-gallon tank is the common choice for whole-home heating, while a 1000-gallon tank is usually used for large homes, farms, high-demand heating, or light commercial use.
The right size depends on your climate, appliance load, available space, delivery schedule, and local code requirements. For installation or replacement, confirm the final tank size with a licensed propane supplier.
Frequently asked questions
How many gallons are in a 20 lb propane tank?
A standard "20 lb" grill cylinder holds about 4.7 gallons of propane (20 ÷ 4.24 lb per gallon). Because the cylinder is rated by the weight of propane it holds, this figure already reflects the usable propane inside — the 80% fill rule is not applied a second time.
How many gallons are in a 30, 40, or 100 lb propane tank?
Roughly 7.1 gallons for a 30 lb cylinder, 9.4 gallons for a 40 lb cylinder, and 23.6 gallons for a 100 lb cylinder, using about 4.24 pounds of propane per gallon at around 60°F.
How many gallons is a 420 lb (120-gallon) propane tank?
This tank is named two different ways, which gives two slightly different figures. By its 120-gallon water capacity and the 80% fill rule it holds about 96 gallons of usable propane (120 × 0.8). By its 420 lb propane rating divided by 4.24 lb/gal it works out to about 99 gallons. The small gap comes from these being two nominal ratings of the same physical tank; naming varies by supplier and manufacturer.
How long does a propane tank last?
It depends on the appliance and how hard it runs. A 20 lb grill cylinder typically lasts roughly 18–20 hours on a medium grill, while a whole-home heating tank is sized for weeks or months between fills. To estimate, divide the usable gallons by your appliance's gallons-per-hour consumption.
Why are propane tanks only filled to about 80%?
Liquid propane expands as it warms — roughly 1.5% for every 10°F rise. Leaving about 20% of the tank as vapor space gives the liquid room to expand and prevents dangerous over-pressure. The 80% limit is part of NFPA 58 (the LP-Gas Code) and is enforced on cylinders by an overfill prevention device (OPD) valve.
Do propane tanks expire or need recertification?
Portable DOT cylinders carry a stamped manufacture date and must be requalified after an initial period before they can be refilled. The qualification and requalification intervals vary by cylinder type and by the test method used, so there is no single universal date. Confirm your cylinder's current status and the applicable requirement with your propane supplier or refiller.
Assumptions and limitations
The figures on this page are approximate planning estimates. Dimensions vary by manufacturer, model, and tank style. Portable-cylinder gallons are calculated from rated propane weight using about 4.24 lb per gallon. Stationary-tank usable capacity assumes the common 80% fill level. Actual fill level and gauge readings can vary with temperature and supplier practice. These numbers are for planning only, not for billing, custody-transfer measurement, installation approval, or safety compliance.
Disclaimer: Propane is a flammable, pressurized fuel. Tank sizing, placement, and filling must follow applicable local codes and be handled by a licensed propane supplier. These estimates do not replace professional advice.
References and cross-checks
Tank dimensions were cross-checked against propane supplier sizing guides and manufacturer specification charts. The 80% fill guidance follows NFPA 58, and portable-cylinder requalification follows U.S. DOT rules under 49 CFR 180.209. Dimensions and usable capacity can vary by manufacturer, tank style, temperature, and local fill practice, so the values above should be treated as planning estimates.
Volume conversions follow standard U.S. customary definitions, as documented on our methodology page.