How the calculator works
Add up the adult length of every fish (length × count), then compare the total with two rules of thumb:
- Inch-per-gallon rule: 1 inch of adult fish per US gallon. The calculator uses the gallons you enter, or works them out from the dimensions (length × width × height ÷ 231).
- Surface-area rule: 1 inch of adult fish per 12 square inches of water surface for tropical tanks, and per 30 square inches for coldwater tanks. The surface is length × width, so this rule needs the dimensions.
Each rule is shown as a percentage of what it allows. Both are rough, and both ignore what actually decides whether a tank works: how much waste each species produces, body shape, behavior, schooling needs, territory and filtration. Use them to catch an obviously crowded plan, then research each species.
Why the two rules disagree
The gallon rule rewards height; the surface rule rewards floor area. A 20 high and a 20 long hold about the same water, but the long tank has 360 square inches of surface against 288, so the surface rule lets it carry 30 inches of fish against 24. Tall tanks look roomy in gallons and score worse on surface. Long tanks are the better buy for most community fish.
How many inches of fish each tank allows
Computed from the usual outside dimensions of standard US tanks, so treat the numbers as upper limits.
| Tank (length × width) | Surface | Inch-per-gallon rule | Surface rule, tropical | Surface rule, coldwater |
|---|---|---|---|---|
| 10 gallon (20 × 10 in) | 200 sq in | 10 in | 17 in | 7 in |
| 20 high (24 × 12 in) | 288 sq in | 20 in | 24 in | 10 in |
| 20 long (30 × 12 in) | 360 sq in | 19 in | 30 in | 12 in |
| 29 gallon (30 × 12 in) | 360 sq in | 28 in | 30 in | 12 in |
| 40 breeder (36 × 18 in) | 648 sq in | 45 in | 54 in | 22 in |
| 55 gallon (48 × 13 in) | 624 sq in | 57 in | 52 in | 21 in |
| 75 gallon (48 × 18 in) | 864 sq in | 79 in | 72 in | 29 in |
| 125 gallon (72 × 18 in) | 1,296 sq in | 118 in | 108 in | 43 in |
Adult sizes of common aquarium fish
These are the commonly cited adult lengths, and the calculator's species list uses them. Individual fish vary, and fish kept well can reach the top of their range.
| Fish | Adult length | Metric | Notes |
|---|---|---|---|
| Ember tetra | 0.8 in | 2 cm | Schooling; keep 6 or more |
| Celestial pearl danio | 1 in | 2.5 cm | Shy; keep 6 or more |
| Neon tetra | 1.5 in | 3.8 cm | Schooling; keep 6 or more |
| White Cloud Mountain minnow | 1.5 in | 3.8 cm | Cool water; keep 6 or more |
| Otocinclus | 2 in | 5.1 cm | Algae grazer; keep in a group |
| Cardinal tetra | 2 in | 5.1 cm | Schooling; keep 6 or more |
| Rummy-nose tetra | 2 in | 5.1 cm | Schooling; keep 6 or more |
| Harlequin rasbora | 2 in | 5.1 cm | Schooling; keep 6 or more |
| Zebra danio | 2 in | 5.1 cm | Very active; keep 6 or more |
| Cherry barb | 2 in | 5.1 cm | Peaceful barb; keep in a group |
| Guppy | 2 in | 5.1 cm | Females larger than males |
| Honey gourami | 2 in | 5.1 cm | Peaceful |
| Panda corydoras | 2 in | 5.1 cm | Bottom schooler; keep 6 or more |
| Platy | 2.5 in | 6.3 cm | Livebearer; breeds readily |
| Betta (male) | 2.5 in | 6.3 cm | One male per tank |
| Bronze corydoras | 2.5 in | 6.3 cm | Bottom schooler; keep 6 or more |
| Tiger barb | 3 in | 7.6 cm | Fin nipper; keep 6 or more |
| Dwarf gourami | 3.5 in | 8.9 cm | Males can be territorial |
| Kuhli loach | 4 in | 10.2 cm | Slender burrower; keep in a group |
| Pearl gourami | 4.5 in | 11.4 cm | Peaceful; needs a larger tank |
| Bristlenose pleco | 5 in | 12.7 cm | Heavy waste for its size |
| Angelfish | 6 in | 15.2 cm | Tall body; eats very small fish |
| Fancy goldfish | 6 in | 15.2 cm | Coldwater; bulky, heavy waste; 6–8 in |
Example: a 29-gallon community tank
The tank is 30 × 12 × 18 inches and tropical. The plan is ten neon tetras, six panda corydoras and one honey gourami.
- Adult inches: 10 × 1.5 + 6 × 2 + 1 × 2 = 29 inches of fish.
- Gallons: 30 × 12 × 18 ÷ 231 = 28.05, so the inch-per-gallon rule allows 28 inches: 103 percent.
- Surface: 30 × 12 = 360 square inches ÷ 12 = 30 inches allowed: 97 percent.
That tank is full by both rules. Adding a pair of angelfish would take it to 41 inches, 146 and 137 percent, and angelfish are known to eat neon tetras anyway.
Tips
- Plan for adult size. Fish in the store are often a fraction of their adult length.
- Respect schooling numbers. Six small schooling fish is a better plan than two of each of three species.
- Stock slowly. Add a few fish at a time to a cycled tank so the filter bacteria can catch up.
- Count your real water. Substrate and rock take up space; use the aquarium volume calculator for the corrected gallons.
Frequently asked questions
How many fish can I put in a 10-gallon tank?
The inch-per-gallon rule allows about 10 inches of adult fish, and the surface-area rule allows about 17 inches for a 20 × 10 inch tank. Six neon tetras (9 inches) come to 87 and 54 percent. Stick to small, peaceful species, and check adult sizes, since many fish are sold as juveniles.
What is the one inch per gallon rule?
One inch of adult fish per US gallon of water. It only works for small, slim fish. Body mass grows much faster than length, so one 6-inch goldfish produces far more waste than four 1.5-inch tetras with the same total length.
What is the surface-area rule for fish tanks?
About 1 inch of adult fish per 12 square inches of water surface for tropical fish, and 1 inch per 30 square inches for coldwater fish such as goldfish. Oxygen enters the water at the surface, so a long, shallow tank scores better than a tall one with the same gallons.
How many neon tetras can go in a 20-gallon tank?
In a 20 long, twelve neon tetras (18 inches of fish) use 96 percent of the inch-per-gallon rule and 60 percent of the surface-area rule. A school of that size fits, but the gallon rule leaves no room for other fish, so the surface-area figure is the one to plan around.
How many goldfish can I keep in a 29-gallon tank?
By the coldwater surface-area rule, a 30 × 12 inch tank allows about 12 inches of fish, which two fancy goldfish at 6 inches already fill. Many keepers give goldfish more room than that, and common and comet goldfish grow too big for most aquariums.
Does the calculator account for my filter?
No. Neither rule of thumb knows about filtration, plants, water-change schedule or how messy each species is. A strong filter and frequent water changes help with waste, but they do not give fish more swimming room.
Last updated October 17, 2026.