How to measure your tank

For a rectangular tank, multiply length × width × height; for a round tank, the calculator uses π × radius² × height. Measure the inside of the glass in inches or centimeters — measuring the outside counts the glass thickness and overstates the volume.

The headline figure is the tank’s full volume. Your actual water volume is always lower — substrate, rocks, décor, and the gap you leave below the rim typically take up 10–15%. We show that adjusted figure too, because it’s the number that matters for stocking and for water-change math.

For a betta, aim for at least 5 gallons — enough water to stay warm and stable and give the fish room to swim. See how to set up a betta tank for the full build, and betta tank mates if you’re planning to add other fish (they need more gallons, not fewer).