Aquarium Volume Calculator
Enter the size of your tank. The calculator gives the volume in gallons and liters and the weight of the water, with an allowance for the space taken up by substrate and decor.
How to use this Aquarium Volume Calculator
- Choose a rectangle or a cylinder and the unit you measured in.
- Measure the inside length and width of the tank, or the diameter of a cylinder.
- Measure the height of the water, not the height of the glass.
- Enter an allowance for substrate and decor.
- Press Calculate for gallons, liters and weight.
How to calculate aquarium volume
Volume is the length times the width times the water height, divided by 231 cubic inches in a US gallon.
- Rectangle: gallons = length x width x height / 231 (inches)
- Cylinder: gallons = pi x (diameter / 2)^2 x height / 231
- Liters = gallons x 3.7854
- Water weight = gallons x 8.34 lb
Worked example
A tank measures 48 x 13 x 21 inches. The volume is 48 x 13 x 21 / 231 = 56.7 gallons, or 214.7 liters. With 10% taken up by gravel and decor the water volume is 51.1 gallons, which weighs 426 lb, or 193 kg. The glass, the stand, the substrate and the equipment add more.
Many tanks are sold by a nominal size, such as a 55 gallon tank, and the real size can differ a little. Water levels are also kept below the rim, so use the height of the water line. The weight matters for the stand and floor. A large tank on an upper floor should be checked by someone who knows the load limits. Use the net water volume when you dose medicines or water treatments, because they are dosed on the water and not on the tank size. A new tank also needs time to cycle before fish go in, and the stocking level depends on the real water volume. Many keepers follow a rule such as an inch of fish per gallon, but this is only a rough guide, since body shape and adult size change the load on the filter. Use the net volume when you pick a heater, because the power needed depends on the amount of water you have to warm, and use the full weight when you check that the stand can bear the load.
Assumptions & limits
- Volume is calculated from the inside dimensions you enter. Glass thickness and trim are not included.
- The decor allowance is a guess. Heavy rockwork can take more.
- Water weight uses 8.34 lb per US gallon. Salt water is a little heavier.
- Tank shapes such as bowfront or hexagon are not covered. Use the rectangle for the nearest box shape.
- The total load on the floor also includes the tank, stand, substrate and equipment.
Frequently asked questions
How do I calculate aquarium volume?
Multiply length by width by height in inches and divide by 231 to get US gallons.
How much does a fish tank weigh?
Water weighs about 8.34 lb per US gallon, and the tank, stand and substrate add more.
Why is real volume lower than the label?
Tanks are not filled to the rim, and substrate and decor take up space.
How do I measure a cylinder tank?
Use the inside diameter and the water height.
What volume should I use for medicines?
Use the net water volume after decor and substrate.
Can I use centimeters?
Yes. Choose centimeters and the calculator converts.
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Last updated: November 2026