Fish tank stocking calculator
Enter the tank volume, the size of its water surface and the adult length of the fish you want. The calculator applies two crude rules of thumb and shows the stricter result. These are old aquarist guidelines, not science: read the limits below before buying fish.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose your units and enter the tank volume and the length and width of the water surface.
- Enter the adult length of one fish of the kind you plan to keep.
- Compare the two rules. The count shown follows the stricter one.
How it works
Rule 1: total fish length in inches ≤ tank volume in US gallons. Rule 2: total fish length in inches ≤ surface area in square inches ÷ 12. The fish count is the lower limit divided by the adult length of one fish, rounded down. Litres are converted at 3.785 L per gallon and centimetres at 2.54 cm per inch.
Worked example
A 20 US gallon long tank, 30 × 12.5 inches: rule 1 gives 20 inches; rule 2 gives 375 ÷ 12 = 31.25 inches. The stricter limit is 20 inches, which is 10 fish of 2 inches each.
Limits of these rules
- They count length only, not body bulk, waste, activity or aggression.
- Both are old, informal guidelines; there is no scientific standard behind them.
- A new tank needs stocking in stages while the filter matures.
- Always check the needs of each species, and keep schooling fish in groups.
Frequently asked questions
Is the inch-per-gallon rule reliable?
No. It is a crude rule of thumb. It ignores that a fat-bodied fish makes far more waste than a slim one of the same length, that some species need space to swim or defend a territory, and that filtration and water changes matter. Treat the number as a ceiling, not a target.
What is the surface-area rule?
It allows about 1 inch (2.5 cm) of adult fish for every 12 square inches of water surface, because oxygen enters at the surface. It suits long, shallow tanks better than the volume rule, but shares the same weakness: it ignores the fish's shape and needs.
Which size should I enter for the fish?
The full adult length stated for the species, not the size in the shop. Fish grow, and tanks are often overstocked because juveniles are measured.
Do these rules work for goldfish or big cichlids?
Poorly. Goldfish and other large, messy species need much more water per fish than the rules say, and aggressive or territorial species need space. Research the specific species.