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Fish Tank Stocking Calculator

Find your tank volume and a safe total fish length using the classic inch-per-gallon rule.

How it is calculated

All three dimensions go in as centimetres: length, width and height, where the height should be the water depth rather than the glass height. Volume is litres = L × W × H / 1000, since 1,000 cubic centimetres make one litre. US gallons are litres × 0.264172, shown with one decimal. The stocking figure follows the old inch-per-gallon rule converted to metric, total adult fish length = litres × 0.67 cm, rounded to a whole number. It is a total length for all fish combined, counted at adult size. The tool takes the full rectangular volume: it subtracts nothing for gravel, rocks or decorations, and it ignores filtration, species and temperature.

Worked example

A tank 100 cm long, 40 cm wide and filled to 45 cm. Volume: 100 × 40 × 45 = 180,000 cm³, divided by 1,000 gives 180 L. In gallons: 180 × 0.264172 = 47.6 gal. Stocking: 180 × 0.67 = 120.6, which the tool shows as 121 cm. That means roughly 121 cm of adult fish length in total, for example about twelve fish of 10 cm, though the rule says nothing about whether a particular species suits the tank.

Try this example →

Common mistakes

  • Entering the outside dimensions in inches or metres. The fields are centimetres; 1 m should be typed as 100.
  • Using the glass height instead of the water level. A tank filled to 5 cm below the rim holds noticeably less, and the volume and stocking number are overstated.
  • Counting the length of young fish bought at the shop. The total should use adult size, which is what the fish will grow to.
  • Forgetting that gravel, rockwork and wood displace water. The tool gives the gross volume, so the real water is lower.
  • Treating the total centimetres as permission to mix any species. Heavy-bodied fish such as goldfish or large cichlids need far more space than their length suggests.

About this calculator

This calculator finds tank volume from its dimensions and applies the traditional “one inch of fish per gallon” guideline (roughly 1 cm per 1.5 liters) to suggest a safe total fish length — a rough surface-area-and-bioload heuristic from classic fishkeeping literature, not a precise biological limit.

Beginner aquarium hobbyists use it as a starting point before stocking a new tank, but experienced keepers and fish stores treat it only as a rough guideline: it breaks down for fish with a lot of body mass relative to length (goldfish, large cichlids), for fish that need more swimming room than the rule implies, and it ignores filtration capacity and bioload, which matter more than raw inches for actual water quality.

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Comments (1)

  • Editorial team

    Even when the total length looks acceptable, a new tank is better stocked in stages. The filter's bacteria need time to grow, so adding a few fish first and the rest weeks later is gentler than adding the whole total at once. Also remember that the tool counts the full box of water. Gravel, rock and decorations take up space, so your real volume and the stocking figure will come out lower. Which species are you planning, and in what order?

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