Fish Stocking Calculator
The "Inch Per Gallon" Rule, Applied Correctly
New aquarists are handed the "one inch of fish per gallon" guideline almost immediately, but it's frequently misapplied — used for a single large goldfish, a heavy-waste cichlid, or a deep-bodied species it was never meant to cover. Applied to what it was designed for — small, slim-bodied community fish like tetras and rasboras — it gives a fast, reasonable starting point for how many adult-size fish a tank can support.
The Formula
Max Fish Count = Tank Volume (gallons) ÷ Average Adult Fish Length (inches)
The result is rounded down, since a stocking count is a ceiling, not a target to round up toward.
Where This Guideline Helps — and Where It Breaks Down
- Community tank planning — sizing a school of small tetras or rasboras for a given tank is exactly the use case this rule was built for.
- Quick sanity checks — before buying fish at a store, a fast division tells you whether the tank at home can realistically support them.
- Where it fails — goldfish, cichlids, and other deep-bodied or heavy-waste species produce far more bioload per inch of length than the rule assumes, and need a species-specific or surface-area-based approach instead.
| Tank volume (gal) | 1-inch fish | 2-inch fish |
|---|---|---|
| 10 | 10 | 5 |
| 20 | 20 | 10 |
| 29 | 29 | 14 |
| 55 | 55 | 27 |
| 75 | 75 | 37 |
Max fish count = tank gallons ÷ average fish length, rounded down.
How to Use This Calculator
- Enter the Tank Volume in gallons — use the Aquarium Volume Calculator first if you only know the tank's dimensions.
- Enter the Average Adult Fish Length in inches for the species you're stocking.
- Select Calculate to see the approximate maximum fish count.
Note: This rule of thumb applies to small, slim-bodied community fish only. Always research the adult size, temperament, and bioload of each species before stocking, and factor in filtration capacity alongside raw tank volume.
Related Calculations
Confirm your tank's true water volume first with the Aquarium Volume Calculator.