Measure length and width of the area to be graveled and determine the desired thickness of the gravel layer. If the area is a circle, measure the diameter instead of length and width. Record all measurements in feet.
Calculate the number of cubic feet of gravel by multiplying the length times width, times depth or thickness. If the area is a circle, divide the diameter by two to obtain the radius length, and then square the radius, multiplying the number by itself. For example, if the radius is 7 feet, the squared number is seven times seven, equaling 49 feet. Once the radius is squared, multiply the number by 3.14 which is phi. The answer to this multiplication is the circle's area. Now multiply the area by the desired depth to obtain the needed cubic feet.
Divide the number of cubic feet by 27 to derive the number of cubic yards. There are 27 cubic feet in one cubic yard.