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Fish Tank Volume Calculator

Aquarium Stocking Calculator

Recommended fish

25

small fish (approx.)

Total fish length

75 cm

≈ 20 inches per 20 US gal

Based on the 1 cm of fish per litre guideline (about 1 inch per US gallon). This is a starting point only. Filtration, fish species, adult size and territory all affect safe stocking. Understock for messy or active fish.

Estimate how many fish your aquarium can hold. Enter the water volume and the average adult fish length and get a recommended number of fish based on the proven stocking guideline.

How many fish can I keep?

A common guideline is 1 cm of adult fish per 1 litre of water, or about 1 inch of fish per US gallon. Divide the water volume by the adult length of your fish to estimate the number you can keep. Treat it as a starting point: filtration, species, adult size and territory all matter, and messy or active fish need more space.

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Frequently asked questions

How many fish can I put in my tank?
As a guide, allow 1 cm of adult fish per litre. A 75 litre tank suits about 75 cm of small fish, for example 25 fish at 3 cm each.
What is the 1 inch per gallon rule?
It suggests 1 inch of adult fish length per US gallon of water. It is the imperial version of the 1 cm per litre guideline.
Is the 1 inch per gallon rule accurate?
It is only a rough starting point. Body shape, waste output, activity level and adult size all change how many fish are safe.
How many fish fit in a 20 gallon tank?
A 20 US gallon (76 litre) tank holds roughly 76 cm of small fish, about 25 tetra-sized fish, with good filtration.
Should I use tank volume or water volume for stocking?
Use the actual water volume, after subtracting substrate and decorations, for the safest estimate.
Does filtration change how many fish I can keep?
Yes. Strong filtration and frequent water changes let you keep a little more, while light filtration means you should understock.

Related guides

Learn the core ideas behind fish tank volume before or after using the calculator.