About StockingCalc
StockingCalc estimates how many inches of fish, and how many adult fish, a tank can hold, and shows each step of the working.
What it does
StockingCalc estimates how many inches of fish, and how many adult fish, a freshwater or saltwater tank can hold. You enter the tank's length, width, and height, the adult size of the fish you want, and your filtration. The calculator returns a maximum, a comfortable number, and the rule it used.
Why it exists
People standing in a fish store ask the same question: how many fish can I put in this tank? The usual answer is a bare number with no working. This site shows every step, so you can check the arithmetic, change an assumption, and see why one rule gives a different answer than another.
How the calculation works
- Gallons
- Gross gallons = length × width × height (in inches) ÷ 231, because a US gallon is 231 cubic inches.
- Substrate and decor
- We take 12% off by default (10% or 15% on request), because sand, rock, and decor displace water. Net gallons = gross × (1 − deduction).
- Gallons per inch
- 1 gal per inch for small, slim freshwater community fish; 3 gal per inch for goldfish, other heavy-waste fish, and any freshwater fish with an adult size of 5 in or more; 5 gal per inch for saltwater.
- Surface area
- As a second check for freshwater, 12 sq in of water surface per inch of tropical fish and 30 sq in per inch for coldwater fish such as goldfish.
- Filtration
- We multiply the result by 0.85 for low, 1.0 for medium, or 1.15 for high filtration. This factor is our own conservative adjustment, not a published standard.
- Fancy goldfish
- The 20 + 10 gal guidance (about 20 gal for the first fish and 10 gal for each additional one) is common hobbyist guidance, which we show next to the 3 gal per inch result.
These are rules of thumb, not science. Bioload, adult size, temperament, swimming space, and the minimum tank size of each species matter more than any inch rule. Adult sizes in our tables are typical hobby figures and vary by strain and by fish. Read the full reasoning, with worked numbers, in aquarium stocking rules explained.
Sources
These are the references we checked the rules and units against. They describe common practice; we do not claim any of them endorses this site. Where our numbers differ, we say so.
- The Spruce Pets, “How Many Fish Should Go in a Tank?”
Describes the one-inch-per-gallon rule and a surface-area rule of one inch of fish per 12 sq in (20 sq in for full-bodied fish), and advises using net gallons and adult size.
- Practical Fishkeeping, “Frequently asked questions on stocking densities”
Recommends 1 in per gallon for tropical and coldwater fish and 1 in per 4 gal for marine fish and invertebrates, and discusses surface-area rules. Our 5 gal per inch saltwater rule is more conservative than their 4 gal, and our 3 gal per inch for goldfish is stricter than their 1 in per gal coldwater figure.
- Wikipedia, “Gallon”
The US liquid gallon is defined as 231 cubic inches, the basis of our volume formula.
- NIST, “Approximate Conversions from U.S. Customary Measures to Metric”
A reference for unit conversions.
Try it
Use the fish tank stocking calculator with your own tank, or start with the rules explained page for the origin and limits of each rule.
— StockingCalc