Aquarium Stocking Calculator
Use this aquarium stocking calculator to work out your tank's volume from its dimensions and get a rough idea of how many fish it can support. It applies the classic inch-per-gallon guideline to usable volume — a useful starting point, though responsible stocking always depends on the specific species, their adult size, and your filtration.
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Calculate tank volume and stocking
Enter values above and press Calculate to see your result.
Formula used
Volume is length × width × height, converted to gallons or litres:
US gallons = (L × W × H in inches) ÷ 231Litres = (L × W × H in cm) ÷ 1000
About 10% is subtracted for substrate, rocks and the gap below the rim to give usable volume. The rough stocking guide is then:
Approx. fish = usable gallons ÷ adult fish length (inches)
This “1 inch of fish per gallon” rule is a beginner heuristic for small community fish only.
Worked examples
Standard 20-gallon. A 24 × 12 × 16 inch tank ≈ 20 gallons; with 2-inch fish that's roughly 8–9 small fish.
Metric tank. A 60 × 30 × 36 cm tank ≈ 65 litres.
Bigger fish, fewer fish. The same tank holds far fewer 4-inch fish than 2-inch fish.
How to use this calculator
- Choose inches (for gallons) or centimetres (for litres).
- Enter the tank's length, width and height.
- Enter the average adult length of your chosen fish.
- Press Calculate for volume and a rough fish count.
- Treat the number as a ceiling, then research each species before stocking.
Common tank sizes
| Tank | Dimensions (in) | Approx. volume |
|---|---|---|
| 10 gallon | 20 × 10 × 12 | ≈ 10 gal / 38 L |
| 20 gallon (high) | 24 × 12 × 16 | ≈ 20 gal / 76 L |
| 29 gallon | 30 × 12 × 18 | ≈ 28 gal / 106 L |
| 55 gallon | 48 × 13 × 21 | ≈ 57 gal / 216 L |
Actual volume is a little less than nominal once glass thickness and the rim gap are included.
Who should use this calculator
New and intermediate aquarists planning a community tank, and anyone who needs the tank's volume for dosing treatments, water changes, or buying a heater and filter sized to the tank.
Why the inch-per-gallon rule is only a guide
The 1-inch-per-gallon rule ignores body mass, waste output (bioload), territory, swimming style and schooling needs. A single 10-inch fish produces far more waste than five 2-inch fish of the same total length, and many species need groups or specific footprints. Use the result as a rough upper limit, then research each species' real requirements.
Stocking responsibly
- Filtration first. Strong, well-cycled filtration supports more fish than the rule suggests; weak filtration supports fewer.
- Adult size, not store size. Stock for the size fish will reach, not the size they are sold at.
- Understock and observe. Add fish gradually and watch water parameters.
Limitations of this calculator
This estimates volume accurately but stocking only roughly. It does not account for species behaviour, bioload, filtration, or aggression. Always treat the fish count as a conservative starting point and confirm with species-specific care guides.
Frequently asked questions
How do I calculate my aquarium's volume?
Multiply length × width × height. In inches, divide by 231 for US gallons; in centimetres, divide by 1000 for litres. This tool does both and subtracts ~10% for usable volume.
How many fish can my tank hold?
As a rough guide, about one inch of adult fish per usable gallon — but this depends heavily on species, bioload and filtration. Research each fish before stocking.
Is the inch-per-gallon rule accurate?
It's a beginner heuristic for small community fish only. It overestimates for large or messy fish and ignores schooling and territory needs.
Why subtract 10% from the volume?
Substrate, rocks, décor and the gap below the rim mean the actual water volume is less than the box dimensions suggest.
Does tank shape matter for stocking?
Yes — surface area and footprint matter for oxygen exchange and territory. Long, wide tanks generally stock better than tall, narrow ones of the same volume.