Pet Calculators
Aquarium stocking.
Enter the tank’s US gallons and each fish’s adult length and count to see the gallons they need under a conservative inch-per-gallon rule and how full the tank is. A rough guide, not a guarantee. Runs in your browser.
Tank and fish
Stocking level
Fill the form and press Calculate.
Worked example
Ten neon tetras (1.5 in adult) in a 20 gallon tank: 10 x 1.5 in x 1 gallon per inch = 15 gallons needed, so the tank is 75% stocked with 5 gallons to spare. Two 5 in fish plus six 2 in fish: 2 x 5 x 1.5 + 6 x 2 x 1 = 27 gallons needed, so a 20 gallon tank is about 135% stocked and overstocked.
How it works
The classic rule of thumb is one inch of adult fish per US gallon, which only works for small, slim fish. To stay conservative this tool scales it by adult length: fish up to 3 in need 1 gallon per inch, fish over 3 to 6 in need 1.5, and fish over 6 in need 2. That adjustment is mine, not a published standard. It counts the adult length, never the size in the shop.
What a rule of thumb cannot see: bioload (goldfish and cichlids make far more waste than tetras), species that need a larger tank or a bigger school, territorial or schooling needs, swimming space, filtration and live plants. Many popular fish are sold too small for tanks they will outgrow. Check each species’ minimum tank size and add fish slowly, testing ammonia and nitrite as the tank cycles. Marine tanks follow different, stricter rules.
Sources and basis (checked 2026-10-05): the inch-per-gallon rule is folk guidance, not a scientific standard. FishBase is a good source for adult fish lengths, which is what you should enter. The length scaling is this tool’s own conservative adjustment.
FAQ
How many fish can I put in a 20 gallon tank?
Under a one inch per gallon rule, about 20 inches of small adult fish, for example 13 neon tetras (1.5 in). Larger fish count more per inch in this tool, and some species need a bigger tank whatever the rule says.
Is the inch per gallon rule accurate?
No. It is a rough starting point that ignores bioload, body shape, species needs and filtration. Use it to catch obvious overstocking, not to approve a tank.
Do I count the size of the fish when I buy it?
No. Always use the full adult length, which for many species is much larger than in the shop.
What happens if a tank is overstocked?
Ammonia and nitrite build up, oxygen falls and stress and disease rise. Signs include cloudy water, gasping at the surface and fighting.
Does the rule work for saltwater tanks?
No. Marine fish and corals are much more sensitive and need species-specific advice.
Disclaimer
A rough planning guide for freshwater fish only, not a guarantee of a healthy tank. Research each species’ adult size, group size and minimum tank size before you buy.