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Aquarium Calculators

Fish Stocking Calculator

How many fish a tank can hold, by the surface area and body length rules.

Enter values

Results update as you type.

Result

Fish this tank can hold

15

By surface area rule
66
By volume rule
15
Total fish length
90 cm
Surface area
4,800 cm²
Litres per fish
12
Advice
Stock gradually over several weeks and let the filter mature between additions

About the Fish Stocking Calculator

How many fish a tank can hold, by the surface area and body length rules.

Also known as: how many fish in my tank, aquarium stocking level, inch per gallon rule, fish capacity calculator.

How to use this calculator

  1. Enter tank water volume, tank length, tank width, adult fish length and the other fields.
  2. The result updates instantly as you type — press Calculate (or Enter) at any time.
  3. Review the breakdown shown under the main result.
  4. Use Copy result or Share to save the answer or send a link with your inputs.

Formula

Surface area rule: 1 cm of fish per 12 cm² of water surface

Worked example

Using the values pre-filled in the calculator above:

  • Tank water volume = 180 litres
  • Tank length = 120 cm
  • Tank width = 40 cm
  • Adult fish length = 6 cm
  • Fish type: Small community fish
  • Filtration capacity = 100 %

the calculator returns:

  • Fish this tank can hold: 15
  • By surface area rule: 66
  • By volume rule: 15
  • Total fish length: 90 cm