Explore fish count, tank volume, or average length allowance under the one-inch-per-US-gallon arithmetic rule. This rule does not establish safe stocking or predict adult fish size.

Explore the one-inch-per-US-gallon arithmetic rule.

This rule does not establish safe stocking or predict fish growth. Check species, adult size, swimming space, filtration and water quality separately.

Related Calculators

Fish Per Gallon Formula

The following formula gives a total arithmetic fish count under the rule, not a density or a safe stocking limit.

N = (TV) / (FS)

Variables:

  • N is the total arithmetic fish count; use floor(N) for a whole-fish count
  • TV is the usable tank volume (US liquid gallons)
  • FS is the average adult fish length (inches)

Under the one-inch-per-US-gallon rule, divide tank volume in US gallons by average adult length in inches. Do not convert inch length to feet: that would multiply the result by 12. For the 55-gallon, 2-inch example, the unrounded count is 27.5 and the whole count is 27 fish. Length density is only a heuristic and cannot establish a safe biological load.

What is Fish Per Gallon?

Fish per gallon is a count density: total fish divided by usable US gallons. The calculation here instead explores a length-based arithmetic rule. Neither count density nor length alone establishes safe stocking. Species, adult size, body shape, swimming space, behavior, filtration and measured water quality require separate assessment.

How to Calculate Fish Per Gallon?

The following steps outline how to calculate the number of fish per gallon.

  1. First, determine usable tank volume (TV) in US liquid gallons.
  2. Next, determine the average size of the fish (FS) in inches.
  3. Next, use the rule N = TV / FS, with FS in inches.
  4. Finally, calculate the total arithmetic count N and round down for whole fish.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

tank volume (TV) = 55 gallons

average fish size (FS) = 2 inches