Fish Tank Stocking Calculator
Built by Johnny Alto · free, no sign-up · updated July 26, 2026
Add fish to see a rough stocking estimate for your tank. This uses the classic one-inch-of-fish-per-gallon guideline as a starting point. It is a guideline, not a guarantee, so treat the result as a sanity check and always research each species' adult size, temperament, and group needs before you buy.
0%of the inch-per-gallon guideline (0 in of fish vs 0 in capacity)
The inch-per-gallon rule is a rough starting point that works best for slim community fish. It ignores body mass, bioload, filtration, and adult behavior, so goldfish, cichlids, and heavy waste producers need far more room than the number suggests. Aim to stay under 100 percent and research each species.
How the stocking estimate works
The calculator adds up the adult length of every fish you plan to keep and compares it to your tank's water volume using the one-inch-of-adult-fish-per-gallon guideline. If your total fish length is under your gallons, you are within the guideline; if it is over, you are overstocked by this measure.
The key word is guideline. The rule was built for slim, peaceful community fish and it says nothing about body mass, waste output, swimming space, or temperament. A six-inch angelfish is not the same load as four small tetras that also total six inches, and a goldfish produces far more waste than its length implies.
Why you should understock
Real stocking depends on filtration, water-change routine, tank footprint, and how each species behaves. Strong filtration and frequent water changes let you push higher; a lightly filtered tank should stay well under the line. Schooling fish also need groups, often six or more, to feel secure, so plan numbers around their social needs, not just the capacity figure. When in doubt, understock: it is more forgiving, healthier for the fish, and easier to keep stable.
Recommended gear
Keep a stocked tank healthy
Frequently asked questions
How many fish can I keep in a 20 gallon tank?
As a rough guideline, about 20 inches of adult fish, such as a school of small tetras plus a few livebearers. It is only a starting point; filtration, waste output, and species behavior matter more than the number.
Is the one inch per gallon rule accurate?
It is a rough starting point that works best for slim community fish. It ignores body mass, bioload, and behavior, so goldfish, cichlids, and heavy waste producers need much more room than the rule suggests.
How many neon tetras can I keep?
Neon tetras reach about 1.5 inches and should be kept in groups of six or more. In a 20 gallon tank a school of 10 to 12 leaves room for a few other small fish.
Should I fully stock to the limit?
No. Leave headroom for growth and bioload and stay under 100 percent of the guideline. Understocking is more forgiving and keeps water parameters stable.
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This calculator gives estimates for planning only; always confirm against your own measurements and species research. Some links are to our buying guides, which contain affiliate links. As an Amazon Associate I earn from qualifying purchases.
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