To calculate how much gravel is needed, measure the width and breadth of the area in feet, and determine the depth desired. Multiply the length times the width times the depth for the number of cubic feet, then divide that number by 27 to determine the needed cubic yards.
The measure of volume used for gravel is a cubic yard. To determine depth, usually measured in inches, use a fraction of one foot in the calculation. DoItYourself.com uses this example: if the area is 36 feet long, 9 feet wide and 3 inches deep, use .25 (1/4 of 1 foot) for the depth. The formula is 36 x 9 x .25 = 81 cubic feet of gravel. To determine how many cubic yards are needed for the job, divide 81 by 27, which is the number of cubic feet in a yard, to arrive at 3 cubic yards for the project.