Reference guide
How to Read a Tap Drill Chart
What tap drill charts mean, why thread engagement matters, and when to verify with your tooling supplier.
Start with the thread callout
A tap drill chart begins with the thread size, such as 1/4-20 UNC or M6 x 1. The first part identifies the major diameter. The pitch or thread count determines how much material remains for the tap to form the internal thread.
Check the drill column, not only the nearest decimal
The listed drill is normally a standard fractional, number, letter, or metric drill. The decimal column helps compare alternatives, but the named drill is what a user usually pulls from the index.
Know when the chart is not enough
Deep holes, hard material, thin walls, plated parts, and forming taps can require a different drill than a simple chart value. When the part matters, make a test hole and verify thread fit before cutting final material.