Fish Tank Stocking Calculator
Max inches of fish, a suggested count, and the rule behind it.
9.1 gal net · max 9 fish · comfortable 7
most adult fish this tank holds by the rule
comfortable: 7 (80%), at 1 in each
- One-inch-per-gallon rule: 1 in of fish per 1 gal.
- 10.4 gal gross, minus 1.2 gal (12%) for substrate and decor = 9.1 gal of water.
- Surface-area check: 20 × 10 = 200 sq in, about 16.5 in of tropical fish at 12 sq in per inch.
A rule of thumb, not science. Adult size, temperament, swimming space, and each species' minimum tank matter more than any inch rule.
How to use this calculator
Enter your tank
Tap a preset or type the length, width, and height of the water space in inches. Decimals are fine.
Describe the fish
Pick freshwater or saltwater, then the adult size of the fish you want (not the size at the store). Switch on heavy-waste fish for goldfish. Set your filter and how much volume the substrate and decor take.
Read the result
The big number is the most inches of adult fish the rule allows. Below it you get a suggested count, a meter that shows how full your plan is, and any warnings. Copy result saves it as text.
The rule we use
- Gross gallons =
length × width × height ÷ 231(231 cubic inches in a US gallon) - Net gallons =
gross × (1 − deduction), where deduction is 10%, 12%, or 15% - Gallons per inch (gpi) =
1,3, or5, from the table below - Max inches =
net ÷ gpi × filtration factor, rounded down to 0.5 in - Suggested fish =
floor(max inches ÷ adult size) - Percent stocked =
planned fish × adult size ÷ max inches × 100
| Rule | Gallons per inch | When it applies |
|---|---|---|
| One-inch-per-gallon rule | 1 | Freshwater, community fish with an adult size under 5 in. Small, slim fish only. |
| Large / heavy-waste rule | 3 | Freshwater goldfish and other heavy-waste fish, or any freshwater fish with an adult size of 5 in or more (plecos, cichlids). |
| Saltwater rule | 5 | All saltwater tanks. |
| Filtration | Factor | Meaning |
|---|---|---|
| Low | 0.85 | Small sponge or an undersized filter |
| Medium | 1 | Filter rated for your tank size |
| High | 1.15 | Filter rated well above tank size, with regular maintenance |
Why we take off 12% for substrate and decor
Sand, gravel, rock, and wood take up room, so the tank holds less water than its label says. A 2 in layer of gravel and a few rocks typically displace 10 to 15%. We default to 12%. Use 10% for a bare or lightly decorated tank and 15% for heavy rockwork.
These are rules of thumb used by hobbyists, not science. Bioload, adult size, temperament, swimming space, and species minimum tank sizes matter more than any inch rule. Treat the result as a ceiling for a mature, well-filtered tank, not a target.
Common mistakes
Counting the juvenile size
A 1 in cichlid or pleco at the store can be 5 to 12 in as an adult. Always enter the adult length, or the count will be wrong by a factor of two or more.
Using gross gallons
A tank labeled 20 gal does not hold 20 gal of water once sand, rock, and decor are in. We take off 12% by default. Use the net number.
Applying the 1-inch rule to goldfish and bulky fish
The one-inch rule was meant for small, slim fish. Goldfish, plecos, and cichlids make far more waste per inch, so we switch to 3 gal per inch for them.
Ignoring schooling minimums
Neon tetras, rasboras, danios, and corydoras are kept in groups of 6 or more. A rule that says 'about 3 fish' does not mean three of a schooling species is a good plan.
Adding all the fish at once to a new tank
A new tank has no beneficial bacteria yet. Adding a full stock at once spikes ammonia. Add a few fish at a time and test between additions.
Quick reference: preset tanks
Maximum inches of adult fish with a medium filter and 12% off for substrate and decor.
| Tank (L × W × H in) | Gross gal | Net gal (12%) | Max in, small community | Max in, heavy / large | Max in, saltwater |
|---|---|---|---|---|---|
| 5 gal (16 × 8 × 10) | 5.5 | 4.9 | 4.5 | 1.5 | 0.5 |
| 10 gal (20 × 10 × 12) | 10.4 | 9.1 | 9 | 3 | 1.5 |
| 20 gal high (24 × 12 × 16) | 19.9 | 17.6 | 17.5 | 5.5 | 3.5 |
| 20 gal long (30 × 12 × 12) | 18.7 | 16.5 | 16 | 5 | 3 |
| 29 gal (30 × 12 × 18) | 28.1 | 24.7 | 24.5 | 8 | 4.5 |
| 40 gal breeder (36 × 18 × 16) | 44.9 | 39.5 | 39 | 13 | 7.5 |
| 55 gal (48 × 13 × 21) | 56.7 | 49.9 | 49.5 | 16.5 | 9.5 |
| 75 gal (48 × 18 × 21) | 78.5 | 69.1 | 69 | 23 | 13.5 |
Frequently asked questions
Is the one inch per gallon rule accurate?
Not really. It is a hobbyist rule of thumb that only works for small, slim fish such as neon tetras. It ignores body bulk, waste output, activity, and adult size. A 4 in goldfish makes much more waste than a 4 in total of neon tetras. Treat the result as a ceiling for a mature, well-filtered tank, not a target.
Does a bigger filter let me add more fish?
A little. A stronger filter handles more waste, so we apply our own adjustment: 0.85 for low, 1.0 for medium, and 1.15 for high filtration. That adjustment is conservative and not from any standard. A filter does not give fish more swimming room or stop territorial fights, so do not use it to justify a crowded tank.
Do I count tail fins?
Measure from nose to the end of the tail for slim fish, which is the length most species charts quote. For long-finned fish such as fancy bettas or goldfish, use body length plus a little, since flowing fins add length but not much waste. When unsure, use the larger number.
How many neon tetras in a 10 gallon?
By the rule, a 10 gal tank (20 × 10 × 12 in) holds 9.1 gal of water, which allows 9 in of fish. Neon tetras reach about 1.5 in, so that is 6 neons. They need a group of 6 or more, so 6 is a school with nothing else added.
Why does saltwater need more water per inch?
It is a convention, and the reasons are practical. Many marine fish are territorial, active swimmers, or both. Salinity, pH, and alkalinity drift faster and are less forgiving, and rock displaces more water. So the saltwater rule is 1 in of fish per 5 gal, five times more water than the freshwater small-fish rule.
How fast can I add fish to a new tank?
Cycle the tank first, which usually takes 4 to 8 weeks. After that, add a few fish at a time and wait 1 to 2 weeks between additions. Test ammonia and nitrite before each batch. If either reads above zero, wait. The bacteria that process waste grow in step with the fish you add.
Should I use gross or net gallons?
Net. Gross gallons are length × width × height ÷ 231. Net is what is left after substrate, rock, and decor take up room. For the default 10 gal tank, gross is 10.4 gal and net at 12% is 9.1 gal. Use 10% for a bare tank and 15% for heavy rock work.