Determine the square footage of the area to be covered by rock. Use a tape measure to find the length and width of the area in feet.
Multiply the length of the area to be covered by rock times the width of the area. For example, an area 20 feet long and 40 feet wide would be 800 square feet.
Divide the total square footage by 100 to determine the number of cubic yards needed. One cubic yard of rock will cover approximately 100 square feet, 2 inches deep. For example, 800 square feet divided by 100 equals 8 cubic yards.