Aquarium Stocking Level Calculator

About this calculator

The inch-per-gallon rule is a rough aquarium stocking guideline stating that a tank can safely hold about one inch of adult fish length per gallon of water — for example, a 20-gallon tank could hold roughly 20 inches of fish, split across several individuals. It is a starting heuristic, not a precise bioload calculation, and it works best for small, slim-bodied community fish rather than large, deep-bodied or heavy-waste species like goldfish or cichlids.

Beginner aquarists use this rule when planning a new tank's stocking list because it is far simpler than calculating actual bioload, filtration capacity and territorial space requirements — but experienced hobbyists treat it only as a rough ceiling, since a tank stocked to the inch-per-gallon limit with large, messy fish can still have dangerously poor water quality even though the "inch count" checks out.

This calculator takes your tank volume and applies the inch-per-gallon rule to estimate how many inches of fish it can safely hold, giving you a quick starting point before you research the specific needs of the species you want to keep.