Fish Tank Stocking Calculator

About the Fish Tank Stocking Calculator

"How many fish in my tank can I keep?" is one of the first questions every new aquarium keeper asks, and one of the most important. Too many fish produce more waste than the filter and the beneficial bacteria can handle, ammonia and nitrite build up, oxygen runs short, and fish become stressed or sick. A sensible first estimate helps you plan purchases before you are standing in the shop.

This fish tank stocking calculator answers the question for one type of fish at a time, using the two classic rules of thumb: the inch-per-gallon rule, based on water volume, and the surface-area rule, based on the area of the water's surface. It reports both results and uses whichever is lower, since that is the factor that will limit your tank.

How to Use the Fish Tank Stocking Calculator

Choose gallons and inches or litres and centimetres.

Enter the tank volume, and its inside length and width, which give the water surface.

Enter the adult length of the fish you want to keep, not the size it is sold at.

Choose tropical or coldwater. Goldfish and other coldwater fish need more surface per inch.

The Formulas

  inch-per-gallon rule:  fish = gallons ÷ adult length (in)
  surface-area rule:     fish = (length × width) ÷ 12 in² ÷ adult length   (tropical)
                         fish = (length × width) ÷ 30 in² ÷ adult length   (coldwater)
  answer = the lower of the two, rounded down

Step-by-Step Example

A 20 gallon tank, 24 by 12 inches, for tropical fish that grow to 1.5 inches.

  volume rule:   20 ÷ 1.5 = 13.3 fish
  surface rule:  24 × 12 = 288 in²;  288 ÷ 12 ÷ 1.5 = 16 fish
  lower result:  13 fish

For coldwater fish of 3 inches in the same tank, the surface rule gives 288 ÷ 30 ÷ 3 = 3.2, so about 3 fish, and the surface is the limit.

Why Two Rules?

The two rules capture different limits. Water volume dilutes waste and buffers changes in temperature and chemistry, so the inch-per-gallon rule is really a waste rule. The water surface is where oxygen dissolves and carbon dioxide escapes, so the surface-area rule is an oxygen rule. A tall, narrow tank may hold many gallons but have little surface; a long, shallow tank has lots of surface but less water. Using the lower of the two respects both.

Limits of the Rules

Both rules were devised for small, slim community fish. They break down for large or heavy-bodied fish. A 10 inch fish has far more mass than ten 1 inch fish, so it produces much more waste, and it needs room to turn and swim. That is why a 10 inch oscar does not belong in a 10 gallon tank, whatever the rule says. For fish over about 4 inches, research the species' minimum tank size instead.

Behaviour matters too. Territorial fish such as many cichlids need space and hiding places; schooling fish such as tetras need a group of at least six to feel secure; active swimmers such as danios need length to swim. These needs can make a tank full well before the numbers say so.

Planning a Healthy Tank

Add fish gradually, a few at a time, so the bacteria that process waste can grow to match. Test ammonia, nitrite and nitrate weekly in a new tank, and change part of the water regularly. A good filter, live plants and careful feeding all increase the margin of safety, but they do not replace sensible stocking.

Gallons, Litres and Metric Tanks

If you work in litres and centimetres, the calculator converts internally: 1 US gallon is 3.785 litres and 1 inch is 2.54 cm. A 75 litre tank of 60 by 30 cm, with fish that grow to 4 cm, comes out at about 12 fish, very close to the 20 gallon example, because the tanks are nearly the same size.

Common Tank Sizes

For quick reference, a standard 10 gallon tank measures about 20 by 10 inches, a 20 gallon long about 30 by 12, a 29 gallon about 30 by 12 but taller, a 40 gallon breeder about 36 by 18, and a 55 gallon about 48 by 13. Long and breeder tanks offer more surface and swimming length for the same volume, which is why they are often recommended for active fish.

Mixing Different Fish

This calculator works for one size of fish at a time. To combine species, work out each group's share: 13 fish of 1.5 inches use the whole of a 20 gallon tank, so a group of 6 uses about half, leaving room for roughly 5 fish of 2 inches. For a full mixed list with waste weighting and a filter check, the aquarium stocking calculator adds everything up as a percentage.

Understanding Your Result

The headline gives the number of fish, the lower of the two rules, rounded down.

The volume rule and surface rule lines show each calculation.

The limit line says whether the volume or the surface is the limiting factor.

The note gives a warning for large fish or a reminder about schooling species.

When Should You Use This Calculator?

Use it before buying fish for a new or existing tank.

Use it to compare tank shapes when choosing a new aquarium.

Use it to check whether a tank suits coldwater fish such as goldfish.

Common Mistakes

Using the shop size. Fish grow; use the adult length.

Applying the rules to big fish. Large fish need far more room than their length suggests.

Ignoring the surface. Tall tanks have less surface for oxygen.

Stocking all at once. Add fish gradually as the filter matures.