Tapping is the operation done for forming internal threads by means of the tool called tap. Before a tap is used for cutting internal threads, a hole is to be drilled. The diameter of the hole should be such that it should have sufficient material in the hole for the tap to cut the thread.

Tap drill size is given by:

Tap drill size = Major diameter - Pitch

Calculation:

In M10 × 1.5, 10 mm is the major diameter of the thread and 1.5 mm is the pitch diameter.

Tap drill size = 10 - 1.5 = 8.5 mm