Determining the volume of a fish tank requires fairly simple math calculations that can be determined by taking the tank's measurements. For tanks that come in basic shapes, such as spheres or rectangles, the process is easier than irregular shapes.
Rectangular and cube-shaped fish tanks are the simplest to calculate. In the case of a cube-shaped tank where all the sides are equal, measure one side and cube the number. For example, if one side measures 2 feet, this equates to the following: 2 x 2 x 2 = 8 cubic feet.
The volume of a rectangular tank can be found by measuring the height, length and width and multiplying these numbers together. For example, a tank with a height of 5 feet, a width of 2 feet and a length of 4 feet would be solved by the following calculation: 5 x 2 x 4 = 40 cubic feet.
For spherical-shaped tanks, you can use the following equation: 4/3 x pi x radius^3. The radius is half the diameter, or the distance from the edge to the center of the tank.
For irregular shapes, you can fill the tank up with water and transfer the water to a large measuring jug that has a volume scale. Simply, repeatedly fill the jug from the fish tank and add the totals to get the volume.