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How to Calculate How Many Gallons in a Tank

| Updated August 11, 2017

Knowing how much water your home aquarium holds is critical for the health of your fish. Some chemicals require a certain number of drops per gallon, and estimating wrong can harm your pets. Fortunately, finding the volume of your tank involves one simple math equation.

Tank Size to Gallons of Water Formula

Determining the amount of water in your aquarium is as simple as knowing that volume equals length times width times height and that 1 cubic inch of water is equivalent to 0.004329 gallons. Therefore, the volume of your tank in gallons of water = length x width x height (measured in inches) x 0.004329.

Practical Application

Measure your tank from the inside with a tape measure, sticking along the edges to get the most accurate measurement of the inside volume of the tank. Oddly shaped tanks may not be easily calculable but their volume should be listed on the packaging provided by the manufacturer. The common rule for keeping fish is 1 gallon of water per inch of grown fish. For example, if you have a black neon tetra, the average adult size of which is 1.5 inches, that fish would need 1.5 gallons of water. If you have three, your tank should be at least 4.5 gallons. Some fish do well in more crowded environments, others need more space. Black neon tetras should be kept in groups of four to six, for instance. Ask your local pet store or ichthyologist if in doubt.


From the inside of the tank, Sarah measures 20 inches across the long side, 10 inches across the short side, and 12 inches from the tank bottom to the top. The volume of Sarah's tank is 20 x 10 x 12 x 0.004329 = 10.3896 gallons. Sarah's tank is just more than 10 gallons. She now knows she can safely keep six black neon tetras in that tank.