Measure the length and the width of your driveway. Multiply the length by the width measurements to figure the area. For example, if the width of the driveway is 18 feet and the length is 60 feet, multiply 18 x 60 to equal 1,080 square feet.
Multiply the area by .33 (converting 4 inches to feet) to figure the cubic feet of the driveway for a 4-inch depth of stone. Using the same example: 1,080 x .33 = 356.4 cubic feet. Multiply the area by .5 (converting 6 inches to feet) to figure the cubic feet for a 6-inch depth of stone. Using the same example: 1,080 x .5 = 540 cubic feet.
Divide the cubic feet by 27 to convert to cubic yards. Using the same examples: 356.4 / 27 = 13.2 cubic yards (4 inches of gravel) and 540 / 27 = 20 (6 inches of gravel).