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Aquarium Stocking Level Calculator

See how many fish your aquarium can hold from its volume and your stocking rule.

How it is calculated

The tool first discounts the rated tank volume by 10 %, because gravel, rocks and decor displace water: usable = V × 0.9, with V in US gallons. It then multiplies by your stocking rule: max inches = usable × R, where R is inches of adult fish per gallon (1 is the classic rule). Finally it divides by the average adult length L in inches and rounds down: fish = floor(max inches ÷ L). Volume, rule and length must all be greater than zero, otherwise nothing is calculated. The result is a rough ceiling for small, slim fish; it ignores filtration, body depth and species temperament.

Worked example

A 20-gallon tank, a rule of 1 inch per gallon and fish that grow to 2.5 inches. Usable volume is 20 × 0.9 = 18.00 gallons (90.00% of the rated volume). Maximum fish length is 18.00 × 1 = 18.00 inches. Dividing by 2.5 inches gives 7.2, rounded down to 7 fish. So about 7 fish of that size is the upper limit under this rule, and a few fewer is a safer start.

Try this example →

Common mistakes

  • Entering litres in the volume box. The field expects US gallons; a 100-litre tank is about 26 gallons, not 100.
  • Using the size of the fish you buy instead of its adult length. A 1-inch juvenile can reach 4 inches or more, which changes the fish count a lot.
  • Treating the inch total as a guarantee. Large, deep-bodied or messy fish produce far more waste per inch than small tetras, so the same total can overload a filter.
  • Mixing different species into one average length. Calculate the big fish separately instead of averaging them with tiny ones.
  • Forgetting that the count is rounded down: if your fish is longer than the total inches, the count shows 0.

Sources

  • Home Aquariums — Connecticut Department of Energy and Environmental Protection

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.

It treats 90% of the tank's rated volume as usable water (substrate and decor take the rest), applies the inch-per-gallon allowance you choose and divides by the average adult fish length, giving an approximate fish count to start from before you research the needs of each species.

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