Drilling Sizes for Holes

The size of hole to be drilled depends upon the purpose of that hole. A hole drilled for a rivet with a specific diameter would differ from those drilled to take a screw thread or the plain shank of a bolt of the same diameter. Similarly, the size of a hole which is to accommodate a shaft will depend on the size of the shaft and on the manner in which the hole/shaft combination is to be used.

Additionally, if the hole is to be reamed, then it must be drilled slightly smaller than its nominal size to allow for the metal removed by the reamer.

Drill sizes (as discussed in the Tools topic) are fixed and can be found on charts that list each standard drill size, together with other columns such as clearance and tapping sizes. These charts may also include equivalent sizes displayed in metric, fractional, letter and in the number/letter system.