Measure the dimensions of the container. For example, if you had a rectangular container, measure its length, width and depth. If you are measuring the capacity of a round container, determine the number of cubic feet it will hold by multiplying 3.1416 (an approximation of pi) by the square of the radius by the container’s depth.
Multiply the dimensions by each other to determine the number of cubic feet each container can hold. A rectangular container of the dimensions 2 feet in length by 2 feet in width and 2 feet deep would thus have a volume of 8 cubic feet. A round container with a radius of 3 feet, and a depth of 2 feet would have a volume of about 3.1416 x 9 x 2 = 56.66 cubic feet.
Multiply the number of cubic feet by 7.47 gallons to determine the number of gallons each container can hold. In the rectangular container example above with a volume of 8 cubic feet, 8 x 7.47 = 59.76 gallons. The volume in gallons of the round container is 56.66 x 7.47 = 423.25 gallons.