Aquarium Stocking Calculator
How many fish your tank can hold, with filtration and water change load.
Enter your values
ⓘ Live results
- Stocking level
- 97% — fully stocked
- Gross volume
- 216 L (57 US gal)
- Net water volume
- 184 L after substrate and decor
- Maximum fish at this size
- 21
- Filter turnover
- 5.4x per hour (aim 4-6x)
- Weekly water change (25%)
- 46 L
Working
- 1120 x 40 x 45 cm = 216 L; net 184 L
- 247.2 in of fish vs 48.5 in capacity
Quick answer
Stocking is limited by filtration and waste, not just volume. The one inch of fish per gallon guide works for small community fish once you subtract substrate and decor volume.
capacity = net gallons x inches per gallon
How to use the Aquarium Stocking Calculator
- 1Measure the tank inside dimensions and use the actual water depth, not the full height.
- 2Enter the adult size of the fish, not the size in the shop tank.
- 3Check the filter turnover is at least four times the net volume every hour.
Worked example
Using the values the calculator loads with:
Inputs
- Tank length120 cm
- Tank width40 cm
- Water depth45 cm
- Adult size of each fish6 cm
- Number of fish planned20 fish
- Filter flow rate1000 litres/hour
Results
- Stocking level97% — fully stocked
- Gross volume216 L (57 US gal)
- Net water volume184 L after substrate and decor
- Maximum fish at this size21
- Filter turnover5.4x per hour (aim 4-6x)
- Weekly water change (25%)46 L
- 120 x 40 x 45 cm = 216 L; net 184 L
- 47.2 in of fish vs 48.5 in capacity
Frequently asked questions
Does the inch per gallon rule work for big fish?
No. It breaks down above roughly 10 cm adult size, because waste output scales with body mass, not length. Use it as a starting sanity check only.
Can better filtration let me stock more?
It helps with ammonia and nitrite, but nitrate build-up and territory still limit stocking. More filtration is not a substitute for water changes.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.