About the Aquarium Stocking Calculator
This aquarium stocking calculator estimates how full your tank is with fish. Enter the tank volume in gallons or liters, how strong your filter is, and up to four species with the number of fish, their adult length and their body type. It returns a stocking level as a percentage of the tank’s estimated capacity, a plain-English verdict, and how many “inches of fish” of room are left.
It is meant for freshwater hobbyists planning a community tank, checking whether there is room for one more school, or sanity-checking a shop’s advice. Using adult size matters: a common pleco or a goldfish sold at 2 inches can grow to 8–12 inches, and stocking for the shop size is the most common beginner mistake.
The method extends the classic “one inch of fish per gallon” rule, which only works for small, slim fish. Bulky or messy species produce far more waste per inch, so their length is weighted up, and a stronger filter raises capacity. It is an estimate, not a guarantee: territorial behaviour, swimming space, water chemistry, plants and your water-change routine all matter, so research each species’ minimum tank size too.
With the default inputs, the stocking level is 88%. Change any value above to recalculate instantly.
How to use the aquarium stocking calculator
- 1Enter the tank volume and choose gallons or liters.
- 2Choose how strong your filtration is.
- 3For each species, enter the number of fish and their adult length.
- 4Pick the body type that best matches each species.
- 5Read the stocking level and room left before buying more fish.
Formula and method
The starting point is the traditional rule of one inch of adult fish per US gallon of water, which suits small, slender community fish. Each species’ total length (number × adult length) is multiplied by a body-type factor — 1 for slim fish, 1.25 for medium-bodied fish such as corydoras and gouramis, and 3 for heavy, high-waste fish such as goldfish, plecos and large cichlids — because a deep-bodied fish has many times the mass and waste of a slim fish of the same length.
Capacity is the tank volume in US gallons (liters ÷ 3.785) multiplied by a filtration factor: 0.75 if the filter is undersized, 1 for a filter rated for the tank and 1.25 for heavy filtration. Stocking level is the weighted bioload divided by capacity. These factors are hobby rules of thumb chosen to be cautious, not scientific constants, so treat the percentage as a guide.
- count
- Number of fish of a species
- adult length
- Full-grown body length in inches (cm ÷ 2.54)
- body factor
- 1 slim, 1.25 medium, 3 heavy or messy
- filtration factor
- 0.75 under-filtered, 1 standard, 1.25 heavy
Worked examples
40-gallon community tank
Ten 1.5-inch neon tetras add 15 inches. Six 2-inch corydoras (12 in × 1.25) add 15, and one 4-inch gourami (× 1.25) adds 5, for a weighted bioload of 35 inches. With standard filtration a 40-gallon tank holds about 40, so it is 87.5% stocked.
100-liter tank with a school of rasboras
100 liters is 26.4 US gallons. Twelve 1.75-inch harlequin rasboras total 21 inches of slim fish, so the tank is about 79% stocked with roughly 5.4 inches of room left.
Two fancy goldfish in 20 gallons
Two goldfish that grow to 6 inches total 12 inches, but at a heavy body factor of 3 their bioload counts as 36 inches. Even with strong filtration (20 × 1.25 = 25), the tank is at 144% — goldfish need much larger tanks.
Frequently asked questions
How many fish can I put in my tank?+
For small, slim fish, a cautious starting point is about one inch of adult fish per US gallon with a filter rated for the tank. Bulky or messy fish need far more water per inch, and every species also has its own minimum tank size and swimming-space needs.
Is the one inch per gallon rule accurate?+
Only roughly, and only for small, slender fish. A 10-inch fish has many times the mass of ten 1-inch fish, and goldfish, plecos and cichlids produce lots of waste, which is why this calculator weights their length more heavily.
What happens if my aquarium is overstocked?+
Waste builds up faster than the filter and beneficial bacteria can handle, so ammonia, nitrite and nitrate rise. Fish become stressed, aggressive and prone to disease. Bigger, more frequent water changes and stronger filtration help, but the real fix is fewer fish or a bigger tank.
Should I add all my fish at once?+
No. Add fish a few at a time over several weeks once the tank is fully cycled, testing ammonia and nitrite after each addition, so the bacterial colony can grow to match the extra waste.
Does a bigger filter let me keep more fish?+
Up to a point. Strong biological filtration handles more waste, so this calculator raises capacity by 25% for heavy filtration. It does not add swimming space or oxygen, and nitrate still has to be removed with water changes.