Tank volume calculator
How many gallons a tank holds, and how much is in it now from the depth of the liquid, for square, upright, horizontal and propane-style tanks.
The tank holds
50.5 gallons
A tank with rounded ends can't be shorter than it is wide. Measure the length from end to end, including the rounded ends.
- In the tank now
- 50.5 gal (100.0%)
- In liters
- 191.1 L
- Capacity in liters
- 191.1 L
- Cubic feet
- 6.75 cu ft
- Weight of the water
- 421 lb
US gallons, from the inside measurements. Water at 8.34 lb per gallon; the tank's own weight is extra.
How to calculate tank volume
Work out the space inside the tank in cubic inches, then divide by 231, the cubic inches in a US gallon. In cubic feet, multiply by 7.48 instead.
- Rectangular: length × width × height
- Round, upright: 3.1416 × radius² × height (radius = half the diameter)
- Round, lying down: 3.1416 × radius² × length
- Rounded ends: the straight part as a lying-down cylinder (length minus the diameter), plus a ball for the two ends: 4 ÷ 3 × 3.1416 × radius³
- Partly full: upright and square tanks scale with the depth; a lying-down tank doesn't, because it's narrow at the bottom and top. The calculator works out the slice of the circle below the liquid line.
Worked example
An upright tank 8 feet across and 10 feet tall: the radius is 4 feet, so 3.1416 × 4 × 4 × 10 = 502.7 cubic feet, and 502.7 × 7.481 = 3,760 gallons. Full of water it weighs about 31,359 pounds before counting the tank.
Fish tank and aquarium gallons by size
Gallons for common inside dimensions. Glass thickness, gravel and the gap at the top mean the water is a little less.
| Length × width × height | Gallons | Liters | Water weight |
|---|---|---|---|
| 20 × 10 × 12 in | 10.4 | 39 | 87 lb |
| 24 × 12 × 16 in | 19.9 | 76 | 166 lb |
| 30 × 12 × 12 in | 18.7 | 71 | 156 lb |
| 30 × 12 × 18 in | 28.1 | 106 | 234 lb |
| 36 × 18 × 18 in | 50.5 | 191 | 421 lb |
| 48 × 13 × 21 in | 56.7 | 215 | 473 lb |
| 48 × 18 × 24 in | 89.8 | 340 | 749 lb |
| 48 × 24 × 24 in | 119.7 | 453 | 998 lb |
| 72 × 18 × 24 in | 134.6 | 510 | 1,123 lb |
Round tank gallons per foot of height
For an upright tank, multiply by the height in feet. For one lying down, multiply by the length.
| Inside diameter | Gallons per foot | Gallons per inch |
|---|---|---|
| 2 ft | 23.5 | 1.96 |
| 3 ft | 52.9 | 4.41 |
| 4 ft | 94.0 | 7.83 |
| 5 ft | 146.9 | 12.24 |
| 6 ft | 211.5 | 17.63 |
| 8 ft | 376.0 | 31.33 |
| 10 ft | 587.5 | 48.96 |
| 12 ft | 846.0 | 70.50 |
Horizontal tank chart: depth to percent full
For a round tank lying on its side with flat ends, any size. Measure the liquid depth with a stick and divide by the inside diameter.
| Depth (share of diameter) | Tank is this full |
|---|---|
| 10% | 5.2% |
| 20% | 14.2% |
| 25% | 19.6% |
| 30% | 25.2% |
| 40% | 37.4% |
| 50% | 50.0% |
| 60% | 62.6% |
| 70% | 74.8% |
| 75% | 80.4% |
| 80% | 85.8% |
| 90% | 94.8% |
| 100% | 100.0% |
Propane cylinder gallons
Propane cylinders are named for the pounds of propane they hold when full. Liquid propane weighs 4.24 lb per gallon (EPA AP-42, Appendix A (liquid propane at 60 °F)).
| Cylinder | Gallons of propane |
|---|---|
| 20 lb | 4.7 |
| 30 lb | 7.1 |
| 40 lb | 9.4 |
| 100 lb | 23.6 |
Water weight from the USGS Water Science School (8.34 lb per gallon).
Tank volume questions
How many gallons is a 36 × 18 × 18 tank?
50.5 gallons, measured inside in inches: 36 × 18 × 18 = 11,664 cubic inches, ÷ 231 = 50.5. A 4 × 2 × 2 foot tank holds 119.7 gallons. With gravel and the water a little below the rim, an aquarium holds somewhat less.
How many gallons of propane are in a 20 lb tank?
About 4.7 gallons. A 20 lb cylinder holds 20 pounds of propane when full, and liquid propane weighs 4.24 pounds per gallon. A 30 lb tank holds about 7.1 gallons, a 40 lb tank 9.4 and a 100 lb tank 23.6. For a large tank sold by the gallon, the gallon size is the whole tank; it's filled to less than that to leave room for the propane to expand.
How do you find the volume of a cylinder in gallons?
Multiply 3.1416 × radius × radius × height, with every measurement in inches, and divide by 231. In feet, multiply the cubic feet by 7.48. An upright tank 8 feet across and 10 feet tall holds 3,760 gallons. The chart above gives gallons per foot for common diameters.
How do I tell how much is in a horizontal tank?
Measure the liquid depth with a stick and enter it as the liquid depth. A round tank on its side is narrow at the bottom, so a quarter of the way up isn't a quarter full: a 4 by 10 foot tank with 1 foot of liquid holds 184 gallons, 19.6% of its 940. Half way up is exactly half full.
How much does a tank of water weigh?
8.34 pounds per gallon, so multiply the gallons by 8.34. The 50-gallon aquarium above holds about 421 pounds of water, before the glass, stand and gravel. Check that the floor or stand can carry it.
How do I measure a tank?
Use the inside measurements if you can: for a round tank, the inside diameter; for a tank with rounded ends, the overall length from end to end. For a pool, use the pool volume calculator, and for pipe, the pipe volume calculator.