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 table below compares 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.
Common propane tank sizes
Portable cylinders are rated directly by propane weight, so their gallons come from dividing pounds by 4.24. Stationary tanks are rated by water capacity, so their usable propane is 80% of that capacity, with propane weighing about 4.24 lb per gallon.
| Tank | Type | Approx. dimensions | Rated capacity | Usable propane | Typical use |
|---|---|---|---|---|---|
| 20 lb (grill cylinder) | Portable cylinder | ≈ 18 in tall × 12 in dia. | 20 lb propane (rated) | 4.7 gal (≈ 20 lb) | BBQ grills, patio heaters |
| 100 lb cylinder | Portable cylinder | ≈ 48 in tall × 14.5 in dia. | 100 lb propane (rated) | 23.6 gal (≈ 100 lb) | Fireplaces, dryers, small appliances |
| 120 gal (vertical) | Stationary ASME | ≈ 4.5 ft tall × 30 in dia. | 120 gal water capacity | 96 gal (≈ 407 lb) | Hot water, dryer, range, supplemental heat |
| 250 gal | Stationary ASME | ≈ 8 ft long × 30 in dia. | 250 gal water capacity | 200 gal (≈ 848 lb) | Home heating in milder climates |
| 330 gal | Stationary ASME | ≈ 8.5 ft long × 36 in dia. | 330 gal water capacity | 264 gal (≈ 1,119 lb) | Whole-home heating, mid-size homes |
| 500 gal | Stationary ASME | ≈ 9 ft 11 in long × 37.5 in dia. | 500 gal water capacity | 400 gal (≈ 1,696 lb) | Whole-home heating, larger homes |
| 1000 gal | Stationary ASME | ≈ 16 ft 2 in long × 41 in dia. | 1000 gal water capacity | 800 gal (≈ 3,392 lb) | Large homes, farms, light commercial |
Portable cylinders show the propane they are rated to hold; stationary tanks show usable propane after the 80% fill rule.
The 80% fill rule (stationary tanks)
Stationary propane tanks are never filled completely with liquid. Liquid propane expands significantly as temperature rises, so tanks are filled to about 80% of their water capacity to leave a vapor space for expansion. This is why a 500-gallon tank holds roughly 400 gallons of propane when "full," and a 1000-gallon tank holds about 800. In hot weather the safety margin matters most, which is why overfilling is dangerous and prohibited.
To estimate usable propane for a stationary tank, multiply its water capacity by 0.8. For a 250-gallon tank that is 250 × 0.8 = 200 gallons of usable propane.
Pounds to gallons conversion
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.
| Propane weight | Approx. gallons |
|---|---|
| 20 lb | 4.7 gal |
| 30 lb | 7.1 gal |
| 40 lb | 9.4 gal |
| 100 lb | 23.6 gal |
| 420 lb | 99.1 gal |
Example: a "20 lb" grill cylinder holds about 4.7 gallons of propane, and a 100-lb cylinder about 23.6 gallons.
Choosing a tank size
Smaller cylinders (20–100 lb) suit grills, fireplaces, and single appliances. The 120-gallon vertical tank covers hot water plus a couple of appliances. The 250 and 330-gallon tanks handle whole-home heating in milder climates or mid-size homes, while 500-gallon tanks are the common choice for whole-home heating in colder regions. The 1000-gallon tank serves large homes, farms, and light commercial use. The right size also depends on your climate, how many propane appliances you run, and how often you want deliveries.
Assumptions and limitations
The figures here are approximate planning estimates. Dimensions vary by manufacturer and model. Portable-cylinder gallons are derived from their rated propane weight at a density of about 4.24 lb/gal; stationary-tank figures assume the standard 80% fill of water capacity. The actual fill level a stationary tank reaches depends on temperature at the time of filling (tanks are filled to a fixed liquid level that corresponds to 80% at a reference temperature), so cold-weather fills may show a lower gauge reading. These numbers are for planning only, not for billing or custody-transfer measurement.
Disclaimer: Propane is a flammable, pressurized fuel. Tank sizing, placement, and filling must follow applicable codes and be handled by a licensed propane supplier. These estimates do not replace professional advice.
References and cross-checks
Standard propane tank dimension references and supplier sizing guides were used to cross-check common 20-lb, 100-lb, 120, 250, 330, 500 and 1000-gallon tank sizes. Commercial propane-supplier charts were used only as non-linked cross-checks. Usable propane depends on the rating system (weight for portable cylinders, water capacity for stationary tanks), the 80% fill rule, propane density (~4.24 lb/gal at ~60°F), temperature, and local fill practice.
Volume conversions follow standard U.S. customary definitions, as documented on our methodology page.