How the inch rule is calculated
Allowance in inches = gallons / N, where N is 1, 2, or 3. Each fish contributes adult length times count. Centimeters are divided by 2.54 before they are added, so the rule stays in inches. A fractional fish is not rounded up: the headcount is the allowance divided by one adult length, then rounded down.
Worked example, 10 gallons, N = 1: allowance is 10 inches. Six fish at 1.5 inches use 9 inches, so 1 inch remains under the loose rule. The same fish on N = 2 have a 5 inch allowance, so they are 4 inches over that stricter rule. A 55 gallon tank at 1 inch per gallon and a 2 inch adult allows 27 fish, because 55 / 2 = 27.5 and the page does not count half a fish.
How many fish per gallon
One inch per gallon is the number people search for. It was written for small community fish, and it falls apart for goldfish, cichlids, and any fish that is tall rather than slim. A single fancy goldfish is commonly given a much larger tank than the inch rule would suggest. If the species grows to 8 inches, an "8 inch" juvenile is not an 8-inch-tank fish. Type the adult length.
Surface area
If you enter the water surface, the page also shows inches allowed by 1 inch of fish per 12 square inches. A 20 inch by 10 inch surface is 200 / 12 = 16.7 inches. That rule is about oxygen at the surface. A lid, a bubbler, and the temperature all change how much oxygen the tank actually holds. It is a second warning light, not a second guarantee.
Get the gallons from the volume calculator if you are not sure of the water volume. Rated gallons on the box are a starting point only. Overstocking shows up as ammonia and as fish that cannot turn around, and neither of those is fixed by the inch count alone.
Frequently asked questions
How many fish per gallon in a tank?
The old rule of thumb is one inch of adult fish per gallon of water. A 10 gallon tank would then allow 10 inches, which is ten 1-inch fish or five 2-inch fish. That rule is a rough ceiling for small, slim fish, not a count of what the tank should hold.
How many fish in a 10 gallon tank?
Under 1 inch per gallon, a 10 gallon allows 10 inches of adult fish. Six fish that stay 1.5 inches are 9 inches, which is just under that rule and over the stricter 1-inch-per-2-gallons rule (5 inches of allowance). Count the adult size, not the size in the shop bag.
Why is the inch-per-gallon rule a bad stocking plan?
A 6-inch slender fish and a 6-inch goldfish do not produce the same waste, and they do not need the same swimming room. Aggression, schooling, and the filter matter. Use the number as a first warning, then look up the adult size and temperament of the species.
What is the surface-area rule?
Another old rule allows about 1 inch of fish per 12 square inches of water surface, because oxygen enters at the surface. A 20 by 10 inch surface is 200 square inches, or about 16.7 inches of fish. It is still a rule of thumb. The calculator shows it only when you enter length and width.