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Fish tank stocking calculator

Estimate how many fish your tank can hold using the inch-per-gallon rule.

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The Fish tank stocking calculator turns Tank size (US gallons), Adult fish length (inches) into Maximum fish, instantly and for free. For instance, with Tank size (US gallons) = 30 and Adult fish length (inches) = 2 it returns Maximum fish = 15.

How to use it

  1. Enter your values: Tank size (US gallons), Adult fish length (inches).
  2. Read the result instantly: Maximum fish.

Frequently asked questions

What does the Fish tank stocking calculator actually compute?

It takes Tank size (US gallons) and Adult fish length (inches) and derives Maximum fish from them. The calculation is live as you type, so the result updates on every change.

What information do I need to provide?

2 values: Tank size (US gallons) and Adult fish length (inches). Nothing else is required — no account, no file upload.

Can you show a worked example?

With Tank size (US gallons) = 30 and Adult fish length (inches) = 2, the calculator returns Maximum fish = 15. Those figures come from running this exact tool, so you can reproduce them by entering the same values.

When would I actually use this?

Day-to-day care: how much to feed at a given weight, whether a size is right for the breed, and what a life stage means in human terms.

What is the most common mistake?

Using the seven-years rule for a dog's age. Ageing is fast in the first two years and then slows, and it slows much more for a small breed than for a large one — the multiplier is not a constant.

How accurate is it, and what are the limits?

Rough guideline only — real stocking depends on species and filtration.

Where do the figures come from, and how current are they?

Portions follow published feeding guidelines by species and weight; age equivalences use the size-dependent curves rather than a flat multiplier. None of this replaces a veterinarian, especially for an animal that is unwell.

Further reading

All guides →
ExplainerHow Many Fish Can a Tank Hold? The Inch-per-Gallon Rule Is WrongEvery stocking calculator implements a length-per-volume rule, and waste output scales with body mass, not length. A fish twice as long makes nearly five times the ammonia. Run through the tool's five methods, the same tank comes back holding anywhere from 6 to 12 fish.How-toHow Much Substrate Does an Aquarium Need? Depth, Weight and Grain SizeThe depth follows the job: a bare display, a planted tank whose roots need room, an under-gravel plate. Then comes the number people are unprepared for — substrate is what puts a filled tank past what a piece of furniture can hold. Plus why fine sand compacts and coarse gravel traps waste.How-toWhat Wattage of Heater Does an Aquarium Need? Volume Is Only Half the AnswerThe same tank needs a 100 W heater in a warm flat and 300 W in an unheated room. The variable is the gap between the water and the room, which most rules of thumb drop entirely. Plus why two smaller heaters beat one big one, and where this calculator's recommendation stops being trustworthy.GuideBar Spacing Is the Cage Rule That Actually MattersA budgie needs bars 10 to 13 mm apart and a macaw 25 to 38. Put the budgie in the macaw cage and it can get its head through — the size is a welfare question, the spacing is a safety one.ExplainerFerguson Zones, and the Lamp Distance They ImplyThe same Ferguson zone 3 puts a compact lamp 18 to 30 cm above the basking site and a 54 W tube 74 to 127. The zone describes the animal; the distance belongs to the lamp, and a mesh screen moves it again.How-toThe IATA Crate Formula an Airline Measures at Check-InLength A + half B, width twice C, height D. The width rule is the one that fails people: a dog with 28 cm shoulders needs 56 cm of interior width, and no crate sold by weight rating tells you that.
Fish tank stocking calculator — OneKitly