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Aquarium Stocking Density Calculator

Enter your tank size and stocking rule to estimate fish capacity.

How it is calculated

The tool multiplies tank volume by the stocking rule: capacity = gallons × rule. The rule is the number of inches of adult fish length allowed per gallon, so the classic one inch per gallon is entered as 1. Remaining space is capacity − current inches, and the share used is current / capacity × 100. Gallons are US gallons and fish are measured in inches of adult body length. The current inches field may be left empty, which counts as 0. Gallons and the rule must be above zero and current inches cannot be negative; otherwise there is no result. Nothing stops the remaining space from turning negative: a minus sign means the tank holds more fish than the rule allows. The rule ignores body shape, waste output, filtration and swimming space, so treat the result as a rough first check.

Worked example

A 20-gallon tank is stocked with fish totalling 14 inches of adult length, using the classic rule of 1 inch per gallon. Capacity = 20 × 1 = 20.0 in. Remaining = 20 − 14 = 6.0 in. The share used is 14 / 20 × 100 = 70.00%. You could therefore add about 6 inches of adult fish, for example two 3-inch tetras, before reaching the limit. If you choose the more cautious rule of 0.5 inch per gallon, capacity falls to 10 in and the same 14 inches would be overstocked.

Try this example →

Common mistakes

  • Entering the rule upside down. A rule of one inch per two gallons is entered as 0.5, not 2; entering 2 doubles the capacity to 40.0 in and gives an overstocked tank a false all-clear.
  • Counting the size fish are in the shop instead of adult length. The rule uses the full-grown size, so a 1-inch juvenile that reaches 6 inches counts as 6.
  • Entering litres. The field is US gallons; a 75-litre tank is about 20 gallons, not 75.
  • Ignoring a negative remaining value. A result such as -6.0 in means 6 inches more fish than the rule allows, and the used share goes above 100% (130.00% for 26 inches in this 20-gallon tank).
  • Using the rule for deep-bodied or messy fish. Goldfish or large cichlids produce far more waste per inch than slim fish, so the same inches need a much larger tank than the simple rule says.

About this calculator

The inch-per-gallon rule is a rough aquarium stocking guideline: 1 inch of adult fish length per gallon of water for smaller freshwater tanks (sometimes 1 inch per 2 gallons for larger or more waste-heavy species). This calculator multiplies your tank's gallon capacity by the rule ratio to estimate total stockable “fish-inches,” then subtracts what you've already added to show remaining capacity.

The rule is intentionally conservative and ignores body mass, bioload, and swimming behavior — a deep-bodied goldfish or an active schooling fish needs far more space than a slim inch-for-inch comparison suggests, so most experienced keepers treat it only as a starting sanity check. Beginner aquarists and pet-store staff use it to avoid the most common overstocking mistakes before doing species-specific research or testing water parameters after adding new fish.

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Comments (1)

  • Editorial team

    Enter the real water volume, not the number on the tank's label. Gravel, rock and decorations take up space, so a tank sold as 55 gallons may hold noticeably less water. The tool then multiplies by your chosen rule, so with the default 55 and 1 inch per gallon the capacity is 55 inches, and 25 remain after 30 inches. Which fish do you keep, and what inches-per-gallon figure has worked for you?

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