What this calculator does
The classic stocking guideline is one inch of adult fish per gallon of water, so a 20 gallon tank supports around 20 inches of fish. Enter the adult length of each fish and the calculator totals them and reports what is left.
It is a rough rule and it fails in predictable ways. It uses length while waste production scales closer to body volume, so a single six inch deep-bodied fish produces far more than six one inch tetras. It also ignores territory, swimming style and schooling needs entirely. Treat it as an upper bound for small peaceful community fish and as inapplicable to anything else.
The formula
The adult lengths entered are added together and compared against the tank volume in gallons, taking one gallon as supporting one inch. The remaining capacity is the difference. The calculation uses adult sizes rather than current sizes, which is the single most important thing to get right when stocking a new tank.
| Term | Meaning |
|---|---|
| Inch per gallon | The stocking rule of thumb, using adult fish length against tank volume. |
| Adult length | Full grown size, which is what to enter. Juvenile size is irrelevant to whether a tank will work. |
| Bioload | The waste burden a fish places on the system, which scales with body mass rather than length. |
The inputs explained
| Field | What to enter |
|---|---|
| Tank size (US gal) | Tank volume in US gallons. Use the actual water volume rather than the nominal tank size, since substrate and decoration displace a noticeable amount. |
| Fish lengths (comma-separated, inches) | Adult lengths in inches, separated by commas, one figure per fish. |
When to use it
Planning a new community tank
Listing the adult lengths of everything you intend to keep before buying anything is the point at which overstocking is easiest to avoid.
Checking whether there is room for more
The remaining capacity figure answers the common question directly, subject to all the caveats about what kind of fish.
Reality-checking a shop recommendation
Fish are routinely sold at juvenile size with adult size unmentioned. Running the adult figures is the check worth doing before buying.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much does a 20 gallon tank hold?
Four stocking lists against the same 20 gallon tank.
| Adult lengths (inches) | Fish inches currently stocked | Room remaining | Number of fish counted |
|---|---|---|---|
| 2, 2, 1.5 | 5.5 in | 14.5 in | 3 |
| 2, 2, 1.5, 3 | 8.5 in | 11.5 in | 4 |
| 3, 3, 3, 3, 3 | 15.0 in | 5.0 in | 5 |
| 2, 2, 2, 2, 2, 2, 2, 2, 2, 2 | 20.0 in | 0.0 in | 10 |
Questions
Is the one inch per gallon rule reliable?
Only as a rough upper bound for small, peaceful freshwater community fish. It uses length while waste scales closer to volume, so it badly understates the burden of large or deep-bodied fish. For anything beyond small community species, species-specific guidance is the only sensible basis.
Should I use adult size or current size?
Adult size, always. Fish are sold young and a common goldfish bought at two inches reaches eight or more. Stocking to current size guarantees the tank becomes overstocked as the fish grow, which is the most common way tanks fail.
Does filtration change how much I can stock?
It helps but it does not rewrite the rule. Stronger filtration processes waste faster, which raises the ceiling somewhat, but it does not create swimming space or territory. Overstocking a heavily filtered tank still produces stressed fish.
What about goldfish?
The rule does not apply to them at all. Goldfish are large, long-lived and produce a great deal of waste, and common goldfish need substantially more than an inch per gallon each. A single fancy goldfish is usually given 20 gallons as a starting point.
For aquatic turtles, which need far more volume per animal, see the turtle tank size calculator. To estimate the weight of a caught fish, see the fish weight calculator.