Dig out the driveway bed if you are building a new drive rather than resurfacing an existing one. This should be done before estimating gravel needs for a new driveway because ground composition and features vary. Until you have the bed dug out it is usually impossible to take accurate depth measurements.
Use a tape measure to determine the width and length of the driveway. For long driveways (several hundred feet) a tape measure is not practical, and you can rent a measuring wheel at a building supply store. Multiply the width by the length to find the area of the driveway in square feet.
Estimate how thick the gravel layers need to be. For a new gravel driveway 10 to 12 inches is average. If you are simply putting down a resurfacing layer, you'll want about 2 inches. To make things easier, convert this to a decimal fraction in feet. For example, 2 inches is 0.17 foot (2 inches divided by 12 inches).
Calculate the cubic volume of gravel needed for a driveway. Multiply the thickness by the square footage to find the volume in cubic feet. Then convert this to cubic yards by dividing the number of cubic feet by 27 (the number of cubic feet in one cubic yard). For example, if the driveway is 8 feet wide and 200 feet long, and you plan on a resurfacing layer of 0.17 feet, you have 82000.17 = 267 cubic feet. Divided by 27 this works out to 9.89 cubic yards (round up to 10 cubic yards).
Estimate the tonnage (weight) of gravel required. Gravel is sold by the ton, and there are about 1-3/8 tons per cubic yard. For 10 cubic yards this works out to about 13.75 tons. When you purchase the gravel, add about 10 percent as a reserve. For this example, that works out to 15.125 tons (round off to 15 tons).