Fish Stocking Calculator
Estimate how many fish fit in an aquarium.
Freshwater ~1, use less for messy fish
Results update as you type.
How it works
Overstocking an aquarium stresses fish and fouls the water, and this calculator gives a rough guide to how many fish a tank can hold using the classic inch-per-gallon rule. You enter the tank volume, the average adult length of your fish, and an inches-per-gallon factor, and it returns the total fish length supported and an approximate maximum number of fish. The traditional rule allows about one inch of adult fish per gallon of water for small freshwater community fish, so a 20-gallon tank holds roughly 20 inches of fish, or about ten 2-inch fish. This is only a starting guideline with important caveats. It works reasonably for small, slender fish but breaks down for large or heavy-bodied fish, which produce far more waste and need more space than their length suggests — a single large fish can require many gallons. It also ignores the tank's surface area (which affects oxygen), filtration capacity, the fish's activity and territoriality, and species compatibility. For messy or active fish, use a lower inches-per-gallon value, and always account for adult size, not the small size fish are sold at. Research each species' specific needs and stock gradually while the tank's biological filter matures. Treat this as a first estimate, not a strict limit. Enter your tank volume and fish length to estimate stocking.
Max fish = (gallons × inches per gallon) ÷ average fish length
The inch-per-gallon rule allows about one inch of adult fish per gallon of water for typical freshwater community fish. Multiplying gallons by the inches-per-gallon factor gives the total fish length the tank supports, and dividing by average fish length estimates the number of fish.
Source:Aquarium stocking guidelines
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter aquarium volume and average adult fish length.
- 2
Adjust the options
Adjust inches per gallon if they apply — sensible defaults are already set.
- 3
Read your result
Approximate max fish updates instantly as you type — no button to press.
Frequently asked questions
What is the inch-per-gallon rule?
It is a rough aquarium stocking guideline allowing about one inch of adult fish per gallon of water for small freshwater community fish. A 20-gallon tank would hold roughly 20 inches of fish. It is a useful starting point for small, slender species but has significant limitations for larger or messier fish.
Why doesn't the rule work for big fish?
The rule assumes waste and space needs scale with length, but larger, heavier-bodied fish produce disproportionately more waste and need far more swimming room than their length implies. A single large fish can require many gallons. For bigger species, the inch-per-gallon rule overstocks badly, so research each fish's specific space and filtration requirements instead.
What else affects how many fish I can keep?
Beyond volume, stocking depends on surface area (which governs oxygen exchange), filtration capacity, the fish's activity level and territoriality, water parameters, and species compatibility. Two tanks of the same volume can safely hold different amounts depending on shape and filtration. Use the rule as a rough guide and adjust for these factors and your specific fish.
Should I use adult size or the size at purchase?
Always use the adult size. Fish are often sold small but grow substantially, and stocking based on their juvenile size leads to a badly overcrowded tank as they mature. Look up the full-grown length of each species and plan around that so the tank remains appropriate once your fish reach their adult size.
How should I add fish to a new tank?
Stock gradually, a few fish at a time, especially in a new aquarium, so the biological filter can develop enough beneficial bacteria to process the waste. Adding too many fish at once can overwhelm an immature filter and cause dangerous ammonia spikes. Cycle the tank first and increase the stock slowly while monitoring water quality.