Tap Drill size charts

The most common way to produce an internal female thread is to drill a hole smaller than the major diameter and then use a tap to produce the thread. This is usually referred to as a tapping drill for the size of thread that needs to be produced. A good way to decide on what diameter drill to choose is to use a tap drill reference chart like the examples below.

Metric coarse tap drill chart

The calculation to determine the correct tapping drill for a 60-degree metric thread is to take the major diameter of the thread and minus the pitch. This leaves us with the core or minor diameter of the screw thread. The charts below shows the recommended tapping drill sizes for metric threads.

Metric fine tap drill chart

UNC tapping drill size chart

A good rule of thumb to select a tapping drill for an imperial screw thread is 85% of major thread diameter for a coarse thread and 90% of major thread diameter for a fine thread. Note that this is for threads with a major diameter under 1”. Over this size, the ratio changes and it is better practice to use a reference chart like the ones provided on this page.