How the aquarium volume is calculated
Volume is the area of the tank's footprint multiplied by its height. For a rectangular tank that is simply:
Worked example. A standard 55-gallon tank measures 48 × 13 × 21 inches on the outside. 48 × 13 × 21 = 13,104 cubic inches, and 13,104 ÷ 231 = 56.7 US gallons (215 litres) if it were filled to the very brim.
Other shapes use a different footprint area, then multiply by height in the same way:
| Shape | Footprint area |
|---|---|
| Cube | side × side |
| Cylinder | π × diameter² ÷ 4 |
| Half cylinder | π × length² ÷ 8 (length of the flat back) |
| Corner, quarter circle | π × radius² ÷ 4 |
| Hexagon | 2.598 × side² |
| Corner pentagon | long side² − (long side − short side)² ÷ 2 |
| Bow front | length × end width, plus the curved segment at the front |
Why the real water volume is smaller
The size on the label is a nominal, brim-full figure taken from the outside of the glass. No tank ever holds that much water. Four things take volume away:
- The glass itself. Outside measurements include the glass. On a 55-gallon tank with 6 mm glass, that alone removes about 5%.
- The gap at the top. Water normally stops 2–3 cm (about an inch) below the rim.
- Substrate. Gravel and sand displace water. Roughly 60% of a gravel bed is solid grain; the rest is water sitting between the grains, which this calculator counts.
- Rocks, wood and equipment. A lightly decorated tank loses about 4%; a tank full of rock can lose 10% or more.
Together these usually remove 15–25%. A "55-gallon" tank typically holds 44 to 48 gallons of water.
Which figure to use
Use the real water volume whenever the number matters for the fish: dosing water conditioner or medication, sizing a heater, checking stocking and planning water changes. Use the brim-full figure only when comparing tanks in a shop, because that is how they are all sold.
US gallons, UK gallons and litres
A US gallon is 3.785 litres; a UK (imperial) gallon is 4.546 litres, about 20% bigger. Aquarium products sold in the UK, Canada and Australia are almost always labelled in litres, while American tank sizes use US gallons. This calculator shows all three so that a dosing instruction written for one system can be followed in another.
Common mistakes
- Dosing medication for the label size. A "20-gallon" tank holding 16 gallons of water gets a 25% overdose if you dose for 20.
- Measuring to the top of the plastic trim. Measure the glass, and measure the water height from the top of the substrate to the water line if you want the real volume.
- Mixing up US and UK gallons. A UK gallon is a fifth bigger than a US gallon.
- Forgetting the sump. If your tank has a sump or a large canister filter, add its water volume to the total.
Questions and answers
How do I calculate how many gallons my fish tank holds?
Measure the length, width and height in inches, multiply the three together, and divide by 231. The answer is the volume in US gallons. For litres, measure in centimetres, multiply, and divide by 1,000.
How many litres are in a gallon of aquarium water?
One US gallon is 3.785 litres. One UK (imperial) gallon is 4.546 litres. A 55 US gallon tank is 208 litres.
Why does my tank hold less water than its label says?
The label is a nominal size based on outside measurements and a brim-full tank. Glass thickness, the air gap at the top, substrate and decor usually take away 15–25% of that figure.
How much does aquarium water weigh?
Fresh water weighs about 8.34 lb per US gallon, or 1 kg per litre. Salt water is about 2.5% heavier. A 55-gallon tank holds around 380–400 lb (175–180 kg) of water once set up.
Should I measure the inside or the outside of the tank?
Either works. If you measure the outside, enter the glass thickness and the calculator removes it. If you measure the inside, set the glass thickness to 0.
Related guides
- How to Set Up Your First Aquarium, Step by Step
- Standard Aquarium Sizes Chart
- How Many Fish Can I Put in My Tank?
- Aquarium Heater Size Guide