Finding the right size drill bit is important for a variety of projects. Pilot holes, dowel holes and countersink holes are common applications that require proper sizing. Drill bit charts are helpful, but different factors come into play that make choosing the proper bit slightly more complicated.
Treat drill bit charts for pilot hole sizes as guidelines only. You should consider other factors when choosing a bit. Variables such as wood density, screw type, gauge and application can mean the difference between splits, cracks or reduced gripping power.
Pilot Holes, for Hard and Soft Woods
Manufacturers base screw charts on the shank of the screw, the smooth surface below the head and above the threads; it's difficult to measure without calipers. The rule of thumb for hardwood is that the shank of the screw should measure the same as the bit. Softwood bits should measure 1/64 inch smaller than the shank.
If You're Not Sure
If you're not sure, err with a slightly larger bit, particularly when dealing with hardwood. Choose a bit 1/64 inch bigger than the shaft.
A Common Example
Woodworkers often use the No. 10 wood screw. According to industry charts, a 3/16-inch bit is the most appropriate size to drill a pilot hole in softwood, and a 7/64-inch bit for hardwood.
Pilot Bit Choice
The familiar twist bit is the best all-purpose bit for drilling pilot holes. You can recognize it by its familiar spiraling threads and somewhat blunt tip. Hardened steel twist bits, recognized by their gold color, are one of the best for hardwood.
Dowel Hole Bits
Dowel holes bits should be the same size as the dowel. For example: one of the most common dowel applications is a 3/8-inch dowel penetrating 3/4-inch material. This application requires a 3/8-inch bit.
Dowel Bit Exceptions
Because of inconsistencies in dowels, it's best to start with a bit that matches the diameter of the dowel and drill a few test holes. If the dowel gets stuck or is otherwise too tight, use bits graduating up in 1/64-inch increments until the dowel fits snugly, but not too tight.
Countersink Pilot Bits
Countersink holes are forgiving. They don't have to be the perfect size, just close enough that the head of the screw is below the surface of the wood. One of the most common holes is 1/4 inch deep, made with a 3/8-inch bit. Almost any bit will suffice for a countersink hole, but the best ones have a flat tip or brad-point bit.
Buttons and Plugs
Fine woodworking often involves the use of furniture buttons or plugs, which fill countersink holes with real wood. These applications must fit perfectly, as with dowels. Buttons and plugs are sized by diameter, typically 1/2 inch, and require a bit of the same diameter. The clean-cutting sawtooth bit is one of the best choices for buttons and plugs.