How aquarium volume is calculated
Volume is the footprint area of the tank × its height. For a rectangular tank in inches:
US gallons = length × width × height ÷ 231
In centimeters: liters = length × width × height ÷ 1000. One US gallon is 3.785 liters.
Other shapes only change the footprint area:
- Cylinder: π × (diameter ÷ 2)²
- Hexagon: 2.598 × side² (for a regular hexagon)
- Corner tank: π × side² ÷ 4 (a quarter circle)
- Bow front: length × width + ⅔ × length × bow depth. The curved part is treated as a shallow arc, which is accurate to within a few percent for typical bow fronts.
Why the real water volume is lower
Tanks are sold by their outer dimensions. The glass thickness, the gap below the rim and the substrate all take up space, and rock and wood displace even more. A "55 gallon" tank (48 × 13 × 21 inches) holds closer to 45–48 gallons of water once set up. Use the actual water volume for dosing medication and fertilizer.
Measuring tips
- Measure inside the glass if you can, for the most accurate result.
- For dosing, the most precise method is to count buckets of known volume while filling.
Frequently asked questions
How many gallons is a 48 × 13 × 21 tank?
48 × 13 × 21 = 13,104 cubic inches ÷ 231 ≈ 56.7 gallons by outer dimensions. It is sold as a 55 gallon tank; with substrate and a gap at the top it holds roughly 45–48 gallons of water.
How do I convert aquarium gallons to liters?
Multiply US gallons by 3.785. A 20 gallon tank is about 76 liters; a 75 gallon tank is about 284 liters.
How much does the water in my aquarium weigh?
Freshwater weighs about 8.34 lb per US gallon (1 kg per liter). The tank, substrate, rock and stand add a lot on top of that; see the aquarium weight calculator.