How much does the tank hold?

How do I calculate the volume of a tank in gallons?

Work out the volume in cubic inches and divide by 231, the cubic inches in a US gallon. A box tank 48 × 24 × 24 inches holds 27,648 cubic inches, or 119.7 gallons. With the sizes in feet, multiply the cubic feet by 7.48.

How do I find the volume of a round tank?

Multiply π × radius² × height. A standing tank 24 inches across and 48 inches tall: 3.1416 × 12 × 12 × 48 = 21,715 cubic inches, which is 94 gallons. A lying tank uses its length in place of the height.

How much is in a horizontal tank that is partly full?

The liquid fills a segment of the circle, so the gallons do not rise evenly with the level. In a lying tank 4 ft across and 10 ft long, half full holds half of its 940 gallons, but 1 ft of liquid — a quarter of the height — holds only about 184 gallons, a fifth of the tank. Enter the liquid level to get the exact amount.

How do I measure the liquid level?

Put a clean stick straight down to the bottom through the fill opening and read the wet mark, or read the tank gauge. Measure the tank from the outside and subtract the wall thickness for a closer answer.

How many liters are in a gallon?

A US gallon is 3.785 liters, and a cubic foot is 7.48 gallons, or 28.3 liters. Switch to Metric to enter the tank in centimeters and get liters.

How many gallons are in each inch of a standing round tank?

Multiply π × radius² by one inch and divide by 231. A tank 36 inches across has a radius of 18, so each inch of height holds π × 18² ÷ 231 = 4.41 gallons. A level of 40 inches therefore means about 176.3 gallons. For a rectangular tank, length × width ÷ 231 gives the gallons per inch.

Is a US gallon the same as an imperial gallon?

No. A US gallon is 231 cubic inches (3.785 liters). An imperial gallon, used in the UK, is 4.546 liters, about 20% larger. This calculator uses US gallons, so convert if a tank is rated in imperial gallons.

How the volume is calculated

A rectangular tank is length × width × height. A standing round tank is the area of the circle, πr², times the height. A lying round tank is the same circle area times the tank length, and when it is part full, the liquid fills a circular segment, so the volume depends on the liquid depth through the segment area r² × arccos((r − h) ÷ r) − (r − h) × √(2rh − h²), with h the depth. The result in cubic inches is divided by 231 for US gallons, or in metric mode cubic centimeters are divided by 1,000 to get liters. The calculator treats the tank as a perfect box or cylinder with flat ends.

Worked example: three tanks

A box tank 60 × 30 × 30 inches holds 233.8 gallons; filled to 18 inches it holds 140.3 gallons, which is 60% of the total because 18 is 60% of 30. A standing tank 36 inches across and 60 inches tall holds 264.4 gallons. A lying tank 48 inches across and 120 inches long holds 940.0 gallons when full, but at a liquid depth of a quarter of the diameter (12 inches) it holds 183.8 gallons, 19.6% of the total, and at three quarters of the depth 756.3 gallons, 80.4%. The level in a lying tank is far from proportional to the contents.

Why the real capacity may differ

The dimensions you measure are rarely the inside dimensions. Subtract the wall thickness on every side for a closer answer. Many real tanks have domed or dished ends, cone-shaped bottoms or internal baffles; these change the volume, and the cylinder formulas on this page ignore them. A tank's nominal rating (for instance, a "55 gallon" drum) is a round figure and may differ from the geometric volume by several gallons. Liquids in the tank also expand when warm, so tanks are not normally filled to the brim.

Common mistakes

Entering the diameter where the radius is expected is the usual slip; this calculator asks for the diameter ("across"), so enter the full width. Mixing inches and feet gives answers that are off by factors of 12, 144 or 1,728. For a lying tank the level is measured from the inside bottom of the circle, not from the ground. And a dipstick reading is only meaningful if the stick is vertical and the tank is level; a tank that tilts by even a little gives a wrong level for a lying cylinder.

Sources: volume of a rectangular prism and a cylinder, and the area of a circular segment, from standard geometry (OpenStax Prealgebra 2e, section 9.6, and OpenStax Precalculus for the segment); the US gallon is defined as exactly 231 cubic inches and 3.785411784 liters (NIST Handbook 44, Appendix C). Related: volume calculator, cubic yard calculator and water heater calculator.