This chart lists the actual dimensions of wood screws and the closest fractional size equivalents. The root diameters are measured at the middle of the threaded portion of the screw. The root diameter on some wood screws tapers along the length of the threads. If drilling a straight pilot hole, we recommend the same drill size as the root diameter in hardwoods and 15% smaller in softwoods. Head diameters listed are standard for Flat Head and Oval-head screws. Round Head screws can be 5% to 10% smaller.
When selecting the wood screw size and length that best suits your requirements, keep in mind that the threaded portion of a wood screw takes up approximately 2/3 of the total length of the screw leaving 1/3 for the shank and head.