How to Calculate a Countersink

By David Weedmark
Wood screws have tapered heads and make countersinking easy.

Countersinks involve drilling two holes for a screw, one for the shank and one for the head. Countersinks are useful for making screws flush with the surface or deeper so you can hide them with a wood plug. The heads of wood screws are tapered in a cone shape. Wood drill bits match this shape with a tapered end. These instructions assume you have a full drill bit collection. For most projects, you can adjust your calculations for drill bits of approximate diameter. For fine, detailed work you may want to purchase additional drill bits.

Known Dimensions

Refer to your screws' packaging to find their dimensions. Skip this section if you don't know the screws' dimensions.

Select a drill bit slightly smaller than the screw’s shank dimension to drill a pilot hole in hardwood. Use a drill bit one size smaller than that for softwood. For example, a 1/4-inch screw would require a 11/64 drill bit for hardwood and a 7/64 drill bit for softwood. A 3/8-inch screw would require a 17/64 drill bit for hardwood and a 7/32 drill bit for softwood. A 1/2-inch screw would require a 11/32 drill bit for hardwood and a 19/64 drill bit for softwood.

Select a drill bit for the countersink that matches the diameter of the screw's head. Then continue to the section below on Calculating Depth.

Unknown Dimensions

Hold the screw's shank against a drill bit to compare the size, continuing until you find a bit that matches the screw's diameter.

Select a bit one size smaller for hardwood and two sizes smaller for softwood.

For the countersink, select a drill bit that's of the same diameter as the screw's head. This is most easily done by placing the bit's flat end on top of the screw's head. If your collection of drill bits is limited, use the closest match, selecting one slightly larger for hardwoods and slightly smaller for softwoods.

Calculating Depth

Measure the length of your screw with a ruler. Apply that measurement to your smaller drill bit if you want the screw flush with the wood. Add any desired extra depth for countersinking. Wrap the bit with a small piece of masking tape at the mark.

Drill the hole into the wood, reducing pressure on the drill as you approach the masking tape. When the tape touches the wood, stop drilling.

Measure the screw head's length from the bottom of the cone to the top of the head. Mark that same distance on the second drill bit with a piece of tape.

Drill the hole for the head of the screw, pressing down gently, until the tape touches the wood.

About the Author

A published author and professional speaker, David Weedmark has advised businesses and governments on technology, media and marketing for more than 20 years. He has taught computer science at Algonquin College, has started three successful businesses, and has written hundreds of articles for newspapers and magazines throughout Canada and the United States.