How to Use the Aquarium Tank Size Calculator
This free tool calculates the exact water volume of your fish tank in both US gallons and litres, updating live as you type. Volume is the number every other decision hangs off — bioload capacity, dechlorinator dosing, medication dosing, and heater or filter sizing all start here.
- Choose your tank shape — rectangular, bowfront, or hexagonal.
- Select your unit — inches or centimetres.
- Enter length, width and height — results appear instantly as you type.
- Set your fill level — most tanks run at 80–90% of height.
How the Calculation Works
For a rectangular tank, volume is straight geometry: length × width × height. In centimetres that gives cubic centimetres, which divide by 1,000 for litres; in inches it gives cubic inches, which divide by 231 for US gallons. The fill-level percentage then scales the height term, because water you don’t add is volume you don’t have.
For a bowfront tank, the curved front adds a roughly elliptical segment to the footprint. The calculator approximates this by adding two-thirds of the bow depth × length × height to the rectangular base volume — the standard approximation used across the hobby, accurate to within a few percent for typical bow curves.
For a hexagonal tank, the footprint is a regular hexagon. Its area is (3√3 ÷ 2) × side² — about 2.598 × side² — multiplied by height and the fill factor.
All results convert both ways: litres alongside US gallons, centimetres alongside inches, so you never have to do unit maths yourself.
How to Interpret Your Results
The number you get is the geometric volume of the dimensions you entered. Real-world water volume is always a little lower: glass thickness steals from every side, and substrate, rock, driftwood and equipment all displace water. Subtract roughly 10–15% for a planted tank with substrate, or about 5% for a bare-bottom setup, to get a realistic working volume.
Use that working volume — not the nominal “sticker” size — for dosing water conditioner and medication, and as the input to stocking decisions. When you move on to stocking, remember volume is only one of three factors that matter: bioload capacity (how much waste your filter and plants can process), filtration performance (turnover and biological media), and swimming terrain (footprint and water-column space for the species you keep). Our Fish Stocking Calculator weighs all three rather than relying on outdated length-per-volume folklore.
Frequently Asked Questions
Should I measure inside or outside the glass?
Outside is easier and is what most people do; the calculator’s result is then the geometric maximum. For a more precise figure, measure inside the glass or subtract roughly twice the glass thickness (commonly 6–12 mm / 0.25–0.5 in per pane) from each dimension.
Why is my real water volume lower than the calculator says?
Glass thickness, substrate, hardscape and equipment all displace water, and most tanks aren’t filled to the brim. Subtracting 10–15% from the geometric volume of a decorated, planted tank is a reliable working estimate.
Are these US gallons or UK (imperial) gallons?
US gallons (3.785 litres). UK imperial gallons are larger (4.546 litres), so if you’re comparing against an imperial figure, the litre value is the safest common ground.
What fill level should I use?
80–90% suits most setups: it leaves space for gas exchange, jumpy fish, and equipment. Open-top planted tanks often run lower; tightly covered tanks can run higher.
Does tank shape change how many fish I can keep?
Yes — significantly. Two tanks of identical volume can have very different footprints, and footprint (swimming terrain) often matters more than raw litres, especially for active or bottom-dwelling species. That’s exactly why our stocking tool considers terrain and bioload, not just volume.