🐠 Aquarium Stocking Calculator
Enter your tank size and the total adult length of your fish to see how full the tank is.
Well stocked
59% of capacity — 10″ of a suggested 17″ (net 17 gal).
The classic guideline is about 1 inch of adult fish per net gallon (roughly 85% of the rated volume once substrate and décor are in). It\'s a rough guide only: fish body mass, waste output, temperament, filtration and surface area matter more than length. Tall, heavy-bodied or messy fish (goldfish, cichlids) need far more room than the rule implies. Always research each species\' adult size and needs. 🔒 In your browser.
How the aquarium stocking calculator works
The best-known stocking guideline is about one inch of adult fish per (net) gallon of water. This tool takes your tank's rated volume, trims it to roughly 85% to account for substrate and décor, and compares your total adult fish length against that net capacity — returning a percentage and a status from understocked to overstocked. It uses the adult size of each fish, not its size today, since fish grow.
The inch-per-gallon rule is only a starting point. A single fish's waste and oxygen demand scale with its body mass, not its length, so tall or heavy-bodied fish (goldfish, cichlids, plecos) need far more room than the rule suggests. Filtration, surface area, temperament and territory all matter. Research each species' real requirements before stocking.
Frequently asked questions
How many fish can I put in my tank?
A rough guide is one inch of adult fish per net gallon (about 85% of the tank's rated volume). A 20-gallon tank gives roughly 17 inches of fish. But body mass and waste matter more than length, so treat it as a ceiling, not a target — and research each species.
Is the inch-per-gallon rule accurate?
Only loosely. It works reasonably for small, slender community fish but badly overestimates capacity for large, heavy-bodied or messy fish. A single goldfish, for example, needs far more than its "inches" imply. Use it as a sanity check alongside species research.
Why use adult size, not current size?
Fish grow, often much larger than the store size. Stocking to their current length leads to a badly overcrowded tank within months. Always plan around the maximum adult size of each species.
What is net vs gross tank volume?
Gross is the tank's rated capacity; net is the actual water volume after substrate, rocks, plants and equipment displace some — usually around 85% of the rated figure. Stocking should be based on net volume, which this tool estimates for you.
What else affects how many fish a tank holds?
Filtration capacity, surface area for gas exchange, the fishes' temperament and territory needs, and your maintenance routine. A well-filtered, larger-footprint tank with peaceful fish holds more than the rule implies; an under-filtered one holds less.