Aquarium Stocking Calculator
How many fish your tank can hold, with filtration and water change load.
Enter your values
Enter the tank length in cm.
Enter the tank width in cm.
Enter the water depth in cm.
Enter the adult size of each fish in cm.
Enter the number of fish planned in fish.
Enter the filter flow rate in litres/hour.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | How many fish your tank can hold, with filtration and water change load. |
|---|---|
| Category | Pets |
| Inputs needed | Tank length, Tank width, Water depth, Adult size of each fish, Number of fish planned, Filter flow rate |
| Main output | Stocking level |
| Formula | capacity = net gallons x inches per gallon |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Tank length(cm)
- Enter the tank length in cm.
- Tank width(cm)
- Enter the tank width in cm.
- Water depth(cm)
- Enter the water depth in cm.
- Adult size of each fish(cm)
- Enter the adult size of each fish in cm.
- Number of fish planned(fish)
- Enter the number of fish planned in fish.
- Filter flow rate(litres/hour)
- Enter the filter flow rate in litres/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.
What do I need to enter into the Aquarium Stocking Calculator?
Just 6 values: tank length, tank width, water depth, adult size of each fish, number of fish planned, filter flow rate. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Aquarium Stocking Calculator work out the 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. It applies the formula capacity = net gallons x inches per gallon and shows the working so you can check each step by hand.
Is the Aquarium Stocking Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.