Fish Tank Volume Calculator
A rectangular aquarium holds a volume equal to length times width times height. Convert that to U.S. gallons by dividing the cubic inches by exactly 231, the legal definition of a U.S. liquid gallon, or to liters using the metric relationship of 1,000 cubic centimeters per liter. Advertised tank sizes describe the gross glass capacity, but the water you actually keep is smaller once you account for the fill line below the rim and the space taken up by substrate and decor. Enter your inside dimensions and a realistic fill level to see both the full and the working water volume.
Fish tank volume formula
Cubic inches = length * width * height
U.S. gallons = cubic inches / 231
Cubic cm = cubic inches * 16.387064
Liters = cubic cm / 1000
Water volume = full volume * (fill percent / 100)
One U.S. liquid gallon is defined as exactly 231 cubic inches, and one inch is exactly 2.54 centimeters, so one cubic inch is 16.387064 cubic centimeters. These are exact definitions, not approximations.
Aquarium volume context
- Measure inside dimensions (the water space), not the outside of the glass, for the most accurate volume.
- The advertised tank size is the gross glass capacity; real water volume is lower after the fill line and decor are accounted for.
- A typical fill level is 90 to 95 percent of full height to leave freeboard below the rim.
- Substrate, rock, and large decorations can displace several percent of the volume; reduce the fill level to approximate this.
- Use net water volume, not advertised size, to size filtration and dose water treatments.
Fish tank volume: frequently asked questions
How do I calculate the volume of a rectangular fish tank?
Multiply length by width by height to get cubic inches, then divide by 231 to get U.S. gallons (1 U.S. gallon is defined as exactly 231 cubic inches). For liters, multiply cubic centimeters by 0.001, since 1 liter equals 1,000 cubic centimeters.
Why is my actual water volume less than the tank dimensions suggest?
Gross tank volume assumes the tank is filled to the very top. In practice you fill below the rim and displace water with substrate, rocks, and decor. Use the fill level percentage input to estimate the realistic water volume your filter and heater must service.
What is a U.S. gallon in cubic inches?
One U.S. liquid gallon is defined as exactly 231 cubic inches by the National Institute of Standards and Technology. This calculator uses that exact factor, so a 20 by 10 by 12 inch tank holds 2,400 cubic inches, or about 10.39 gallons full.
Does glass thickness change the volume?
Yes, slightly. Measure the inside dimensions (the water-holding space) rather than the outside of the glass for the most accurate result. For thick acrylic or large tanks the difference can be a gallon or more.
How does tank volume relate to stocking?
Many aquarists use net water volume, not advertised tank size, as the basis for stocking and filtration decisions. Knowing the real liters or gallons lets you size a filter for adequate turnover and dose water treatments at the correct concentration.
Official sources
- National Institute of Standards and Technology: Office of Weights and Measures (U.S. gallon and inch definitions).
- National Institute of Standards and Technology: NIST unit reference.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.