Aquarium Stocking Calculator
How many fish can your tank hold? Add the fish you keep or plan to keep, and see how full the tank is by two classic stocking rules, adjusted for body shape, plus warnings for schooling fish and fish that outgrow the tank.
How to use this calculator
- Enter the tank's length, width and height. The calculator uses them for the water volume, the water surface area and the tank length check.
- Set how much space decor takes up. 15% is a sensible default for gravel, plants and a few rocks or pieces of wood.
- Add each species. Pick a common fish from the list to fill in its adult size and body shape, or choose "Other fish" and type the adult length yourself. Always use the adult size, never the size in the store.
- Read the stocking level and the warnings. The headline uses whichever rule is stricter for your tank.
How the stocking level is calculated
There is no exact formula for how many fish a tank can hold. Waste output, oxygen use, territory and swimming space all matter, and they depend on the species. This calculator uses two long-standing hobby rules side by side, and adds a correction for heavy-bodied fish, because the rules on their own treat a chunky fish the same as a slim one.
Body factor: slender = 1.0, full-bodied = 1.67
- Inch per gallon. One inch of adult fish per gallon of real water. It was meant for small, slender fish, and works reasonably there. LiveAquaria suggests subtracting about 15% of the tank volume for decor before applying it.
- Surface area. 12 square inches of water surface per inch of fish. Oxygen enters at the surface, so a long, shallow tank holds more fish than a tall, narrow one of the same volume.
- Body factor. Some versions of the surface area rule allow 20 square inches per inch for wide-bodied fish instead of 12. That is 1.67 times as much, so full-bodied fish count 1.67 times their length here.
- Tank length check. A common guideline is a tank at least six times the adult length of the largest fish, and at least twice its length from front to back and top to bottom.
Adult sizes in the preset list are maximum standard lengths (snout to the base of the tail) from Seriously Fish species profiles. Many fish in aquariums stay smaller, so the result leans to the cautious side.
Worked example
A 29 gallon tank (30 × 12 × 18 in) holds 28.1 gallons by volume, or about 23.8 gallons after 15% for decor. Its surface is 30 × 12 = 360 square inches.
Stock: 10 neon tetras (1.2 in, slender) = 12 in, plus 6 harlequin rasboras (1.8 in, slender) = 10.8 in. Total adjusted length: 22.8 in.
Inch-per-gallon: 22.8 ÷ 23.8 = 96%. Surface area: 360 ÷ 12 = 30 in of capacity, so 22.8 ÷ 30 = 76%. The stricter figure, 96%, puts this tank at "fully stocked": fine with regular maintenance, but no room for more.
Stocking levels explained
| Level | What it means |
|---|---|
| Under 80% | Lightly to moderately stocked. Room for more fish, and the most forgiving if something goes wrong. |
| 80 to 100% | Fully stocked. Fine with a good filter and regular water changes. |
| 100 to 130% | Above the classic guidelines. Only for experienced keepers with strong filtration, plants and frequent water changes. |
| Over 130% | Overstocked. Expect poor water quality, stress and aggression. |
Adult sizes of popular fish
| Fish | Max standard length | Notes |
|---|---|---|
| Ember tetra | 0.8 in (20 mm) | Shoaling |
| Neon tetra | 1.2 in (30 mm) | Shoaling |
| Cardinal tetra | 1.4 in (35 mm) | Shoaling |
| Harlequin rasbora | 1.8 in (45 mm) | Shoaling |
| Zebra danio | 2.0 in (50 mm) | Shoaling, very active |
| Cherry barb | 2.0 in (50 mm) | Shoaling |
| Panda corydoras | 2.0 in (50 mm) | Shoaling, bottom dweller |
| Platy | 2.0 in (50 mm) | Livebearer, breeds readily |
| Guppy | 2.4 in (60 mm) | Livebearer, breeds readily |
| Betta | 2.8 in (70 mm) | One male per tank |
| Dwarf gourami | 3.5 in (88 mm) | Males can be territorial |
| Kribensis | 3.9 in (100 mm) | Territorial when breeding |
| Bristlenose pleco | 4.9 in (125 mm) | Heavy waste producer |
| Angelfish | 5.9 in (150 mm) | Tall body, eats small fish like neons |
Frequently asked questions
Is the one inch per gallon rule accurate?
It is a rough starting point for small, slender freshwater fish, and nothing more. It ignores body bulk, so a 3 inch goldfish counts the same as three 1 inch tetras even though it produces far more waste. It also ignores tank shape, filtration and behavior.
Should I use the size of the fish when I buy it?
No. Always use the adult size. Many fish are sold as juveniles at a fraction of their final length, and a tank that looks empty today can be badly overstocked in a year.
Why does the calculator warn about schooling fish?
Shoaling species such as tetras, rasboras, danios and corydoras are stressed when kept alone or in pairs. A group of at least six is the common minimum, and larger groups usually show more natural behavior.
Can I stock above 100 percent with a big filter?
Experienced keepers sometimes do, with strong filtration, heavy planting and frequent large water changes. Filtration does not add swimming space or reduce aggression, though, and an overstocked tank has very little margin if a filter fails.
How fast can I add fish to a new tank?
Add fish gradually, roughly a quarter of your planned stock at a time, so the filter bacteria can catch up with the extra waste. Test ammonia and nitrite before each new group.
Related calculators
- Aquarium volume calculator: gallons and liters for any tank shape
- Filter flow calculator: is your filter big enough for this stock?
- Water change calculator: keep nitrate down in a busy tank
Sources: Inch-per-gallon rule, 15% decor allowance, 12 sq in surface area rule, tank length guideline and gradual stocking from LiveAquaria, "Is the one inch per gallon rule the gold standard?". Wide-bodied surface area figure from MonsterFishKeepers discussion of the rule. Adult sizes from Seriously Fish species profiles. Stocking bands are TankMath's own guide.
Last updated: October 2026