Fish Stocking Calculator (1-inch rule)
Calculate your tank's stocking level and maximum fish count using the conservative 0.5 cm fish per liter guideline.
Results
What is it?
The Fish Stocking Calculator uses the conservative 0.5 cm of fish per liter guideline (equivalent to ~1 cm per 2 liters) to calculate what percentage of your tank's capacity is in use and the maximum number of fish of a given size you can keep.
How to use
Enter tank volume, select the fish size category, and enter how many fish you plan to keep. The stocking percentage shows how loaded your tank is: under 80% is healthy, 80-100% is stocked, over 100% is overstocked.
Example scenario
A 200L tank stocked with 10 medium fish (10 cm avg): total fish length = 100 cm. Max capacity = 200 x 0.5 = 100 cm. Stocking level = 100%. At capacity — no room to add more fish.
Pro tip
The 1-inch-per-gallon rule (or its metric equivalent) is a rough baseline that ignores bioload, fish aggression, territory requirements, and waste production. Heavy waste producers like goldfish and cichlids need far more space per inch. Heavily planted tanks and oversized filtration allow higher stocking. Always research the adult size of fish before buying — juvenile fish at the pet store may be 1 cm today and 30 cm at maturity.