Calculating volume can be useful when trying to determine the capacity of a fish tank, gas tank or any container intended to carry liquids. Rectangular containers, such as a fish aquarium, and cylindrical containers, such as oil barrels, are both routinely used to contain fluid. The volumes of these containers are calculated in two very different ways, and require independent formulas.

### Rectangular Container

Measure the container using inches as the unit of measure. You must know the exact length, width and depth.

Multiply the length, width and height to find the volume of the container in cubic inches. The formula is depth x length x width = cubic inches.

Divide the answer from step 2, or the measure of cubic inches, by 12 to convert to cubic feet. The formula is cubic inches / 12 = cubic feet

Multiple the number of cubic feet by 7.47 to calculate the total volume in gallons. This number is used because it is the number of gallons contained in a single cubit foot. The formula is cubic feet x 7.47 = gallons.

### Cylindrical Container

Measure the container using inches as the unit of measure. You must know the exact measure of both the depth and the radius. The radius is half the distance from one side of the circular opening to the other.

Multiply pi by the radius squared, times the depth, to find the volume in cubic inches. Using a calculator, this can be accomplished using the pi button. For manual calculation, use 3.1417 for pi. The formula is pi x r squared x depth = cubic inches.

Divide this answer by 12 to convert to cubic feet. The formula: cubic inches / 12 = cubic feet.

Multiple the number of cubic feet by 7.47 to calculate the total volume in gallons. The formula is 7.47 x cubic feet = gallons.