The gallons in a fish tank make up the volume of the tank. This is useful information in determining how much water is needed to fill the tank. The volume is also useful if there are any additives needed for the water, which you can measure based on the volume of the fish tank.

Measure from the bottom of the fish tank to the top. This is the height. For example, a fish tank might have a height of one and a half feet.

Measure from the front of the fish tank to the back of the fish tank. This is the width of the fish tank. For example, a fish tank could have a width of one foot.

Measure across the front of the fish tank. This is the length of the fish tank. For example, a fish tank might have a length of four feet.

Multiply the height by the length by the width. In the example, multiply 1.5 feet by one foot by four feet, which equals a volume of six cubic feet.

Multiply the cubic feet by 7.48051948. In the example, six cubic feet times 7.48051948 equals 44.8831169 gallons.