Knowing how many fish your tank can safely hold prevents overcrowding, poor water quality, and stressed fish. This calculator applies the classic inch-per-gallon rule of thumb alongside a surface-area cross-check, so you get a sanity check that accounts for tank shape as well as volume.

How the Fish Tank Stocking Calculator works

The calculator sums the adult length of every fish you list (size in inches × quantity) and compares that total against two independent capacity estimates. The first is the widely cited inch-per-gallon rule: 1 inch of fish per gallon of water. The second is a surface-area cross-check based on oxygen exchange — roughly 1 inch of fish per 12 square inches of water surface. Whichever rule gives the lower capacity is the binding constraint, and the status (Understocked, Adequately Stocked, or Overstocked) is based on your total fish inches relative to that binding capacity.

Inputs and what they mean

Tank size in gallons is the primary input. Tank shape (Standard/Long vs. Tall/High) estimates your water's surface area when you haven't measured it directly — a wider, shallower tank has more surface area per gallon than a narrow, tall one of the same volume, so it scores better on the surface-area rule even though the inch-per-gallon rule sees them as identical. If you know your tank's actual surface area (length × width at the waterline, in square inches), choose Custom and enter it directly for a more precise cross-check. On the Add Fish tab, enter each fish's full adult size (not its size at purchase) and how many you have of that size.

Limits and edge cases

Both rules are rough starting points, not hard limits, and they don't account for bioload differences between species. Goldfish and cichlids, for example, produce far more waste per inch of body length than slim community fish like tetras or rasboras, and typically need much more room than either rule suggests. Strong filtration and aeration can safely support higher stocking than the surface-area rule implies, while heavily planted or poorly filtered tanks may need to stay well under both limits. This calculator is a sanity check, not a substitute for researching the specific needs of the species you keep — check adult size, temperament, and bioload for each species before finalizing your stocking plan.