Aquarium Fish Stocking Calculator
Estimate how many fish a tank can hold using the classic inch-per-gallon rule — with clear guidance on its limits and the modern factors that matter more. Runs in your browser.
The 1-inch-per-gallon rule is a rough beginner guideline only. It ignores body mass (a chunky fish needs far more than a slim one of the same length), bioload, filtration, surface area, territory, and species behavior. Modern stocking tools (e.g. AqAdvisor) weigh filtration and species compatibility. Always research each species' adult size and needs; understock if unsure. Not a substitute for species-specific guidance.
About this tool
The oldest aquarium stocking guideline is 'one inch of fish per gallon of water,' and this calculator applies it: divide your tank volume by the adult length of your fish to get a rough maximum count. It is a useful first sanity check for a beginner community tank of small fish — but it is genuinely just a rule of thumb, and the tool is upfront about that. The rule ignores almost everything that actually determines safe stocking: a fish's body mass (a stocky goldfish produces vastly more waste than a slender tetra of the same length), the biological load on the nitrogen cycle, your filtration capacity, the water surface area for gas exchange, swimming room and territory, and species temperament and compatibility. That is why modern stocking tools like AqAdvisor model filtration and species behavior rather than length alone, and why experienced keepers research each species' adult size and needs and tend to understock. Treat the number here as a starting estimate that says 'no more than roughly this,' then verify against species-specific guidance. Everything runs in your browser.
How to use it
- Enter your tank volume in gallons or liters.
- Enter the adult length of the fish you plan to keep.
- Read the rough maximum number of fish.
- Verify against each species' real needs — and lean toward understocking.
Frequently asked questions
- What is the inch-per-gallon rule?
- A beginner guideline that allows about one inch of adult fish body length per gallon of water. A 20-gallon tank could hold roughly 20 inches of fish — for example ten 2-inch fish. It is a starting estimate, not a precise limit.
- Why is the rule unreliable?
- It only counts length, ignoring body mass, waste output (bioload), filtration, surface area, and behavior. A 10-inch goldfish needs far more than a 10-gallon tank, and aggressive or territorial species need space the rule never captures. Use it as a ceiling, not a target.
- What matters more than the rule?
- Filtration capacity and the nitrogen cycle, surface area for oxygen exchange, each species' adult size and waste output, swimming and territorial needs, and compatibility. Tools like AqAdvisor model filtration and species; experienced keepers research and understock.
- Should I use adult or current size?
- Always the adult (full-grown) size. Fish are usually sold young and small; stocking for their current size leads to a badly overcrowded tank as they grow, which stresses fish and crashes water quality.
- Is understocking better?
- Generally yes. A lightly stocked tank has more stable water chemistry, more oxygen, less aggression, and more margin for error — especially valuable for beginners and while a tank is still maturing.
- Is anything uploaded?
- No. The calculation runs entirely in your browser with no network request.