Boring Chart for Wood Screws
How to Use this chart:
The table of Wood Screw Specifications simplifies the selection of the bit or drill size best suited to your requirements. The fractional equivalents and undersize/oversize decimals indicate how close a bit of given fractional size will bore to the actual screw dimension, and whether the fit will be snug or loose. In selecting a tool size for the pilot hole (for the threaded portion of the screw), note that Root Diameters are average dimensions measured at the middle of the threaded portion. On some screws the root diameter tapers slightly from the end to the smooth shank. It is usually good practice to bore the pilot hole the same size as the Root Diameter in hardwoods, and about 15% smaller in softwoods.
The Shank Diameters shown below are standard specifications subject to tolerances of plus 0.004" and minus 0.007."
Maximum Head Diameters apply to Flat Head and Oval Head Wood Screws. Head sizes run 5% to 10% smaller in Round Head Screws.
Number of Threads per inch is rounded to the nearest whole number; this is especially noticeable in the larger sizes.