Pet Calculators
Aquarium volume.
Length times width times height gives the glass volume, but real water is less. Enter the tank size and how full it is kept to get the gallons or litres that actually go in, and what the water weighs. Runs in your browser.
Tank size
Volume
Fill the form and press Calculate.
Worked example
A 48 x 13 x 21 in tank: 48 x 13 x 21 = 13,104 cubic inches, and 13,104 / 231 = 56.7 US gallons to the rim (214.7 L). Filled to 90% with 5% displaced by sand and rock, the water is 56.7 x 0.9 x 0.95 = 48.5 US gallons (183.6 L), which weighs about 405 lb (184 kg) before the glass, stand and sand.
How it works
For a rectangular tank, volume is length x width x height. In inches, divide cubic inches by 231 for US gallons (1 US gallon = 231 cubic inches = 3.785 L); in centimetres, divide cubic centimetres by 1,000 for litres. A cylinder uses pi x radius squared x height. One imperial (UK) gallon is 4.546 L.
Water weighs about 1 kg per litre (about 8.34 lb per US gallon). Add the weight of the glass, stand, substrate and rock: a filled tank is much heavier than the water alone, so check the stand and floor rating. Bowfront and corner tanks have curved sides, so this gives an upper estimate for them: use the maker’s listed volume when you have it.
Sources and basis (checked 2026-10-05): the US gallon definition (231 cubic inches) per NIST Handbook 44 and the density of fresh water (about 1 kg/L at room temperature). Fill and displacement percentages are your estimates.
FAQ
How many gallons is my fish tank?
Multiply length x width x height in inches and divide by 231. For example 48 x 13 x 21 inches is about 56.7 US gallons, or about 48 gallons of actual water at a typical fill level and with substrate.
How do I calculate aquarium volume in litres?
Multiply length x width x height in centimetres and divide by 1,000.
How much does a fish tank full of water weigh?
About 8.34 lb per US gallon (1 kg per litre) for the water, plus the tank, stand, substrate and decor.
Why is real water volume lower than tank volume?
Water is kept below the rim and substrate, rock and decor take up room, so the real volume is often 10 to 20% below the glass volume.
Does a 55 gallon tank hold 55 gallons?
Rarely. A 55 gallon tank is rated at its glass volume. With a normal fill level and substrate it holds about 45 to 50 gallons of water.
Disclaimer
Estimates from the dimensions you enter. Use the manufacturer’s listed capacity when you have it, and confirm that your stand and floor can carry the full weight before filling a large tank.