Work out exactly how much water your aquarium holds, in US gallons, litres and UK gallons, for rectangular, bow-front, cylinder, corner and hexagon tanks. The calculator also shows how much water is really left after substrate and hardscape, and what the tank weighs once it is full.
How aquarium volume is calculated
- Rectangle: length × width × height. In inches, divide by 231 for US gallons; in centimetres, divide by 1,000 for litres.
- Bow-front: the rectangular part plus the bow, treated as half an ellipse.
- Cylinder: π × radius² × height. Corner: a quarter of that, using the side length as the radius. Hexagon: 2.6 × side² × height.
- Actual water leaves a gap below the rim, then subtracts the substrate and the share of space taken by rock and wood.
- Weight: water is 1 kg per litre (8.34 lb per US gallon). Substrate, rock (~2.5 kg/L) and glass (~0.18 kg per litre of capacity) are added as estimates.
A worked example
A 24 × 12 × 16 inch tank holds 19.9 US gallons (76 L), a standard “20 high”. With the water an inch below the rim, 2 inches of sand and about 10% rock, only about 14.6 gallons is actually water. Filled, it weighs around 217 lb, roughly 108 lb on every square foot of the stand.
Plan the setup
- Make sure the stand can take it: load ratings and levelling and DIY stands that hold the weight.
- Ideas by size: betta setups from 5 gallons up.
These calculators use standard formulas and common aquarium guidelines. Fish, plants and rooms vary, so treat the results as a starting point and watch your tank.
Frequently asked questions
How do I calculate the gallons in a fish tank?
Measure the inside length, width and height in inches, multiply them, and divide by 231 for US gallons. For litres, measure in centimetres, multiply and divide by 1,000.
How much does a 20 gallon aquarium weigh?
Full, a 20 gallon tank with substrate and some rock weighs roughly 200–225 lb. Water alone is about 8.3 lb per gallon, and substrate and rock add a lot.
Why is the real water volume lower than the tank size?
Tanks are rarely filled to the brim, and substrate, rock and wood take up space. In our 20 gallon example only about three-quarters of the tank is water, which matters for medication doses and stocking.
Can my floor hold a big aquarium?
It depends on the floor, so use the filled weight from the calculator. Placing the tank near a load-bearing wall and across the joists helps, and for large tanks, or if you have any doubt, check with a builder.
Is my information stored?
No. Everything is calculated in your browser, and nothing you type is saved or sent anywhere. The share link keeps your numbers in the part of the web address after the # sign, which browsers do not send to the website.