Drill Size for Tapping

The tap drill size determines the percentage of thread engagement in the tapped hole. A 75% thread engagement is the standard recommendation for most applications — it provides over 95% of the theoretical pull-out strength while being significantly easier to tap than a 100% thread.

For metric threads, the formula is simple: Drill diameter = Major diameter − Pitch. For example, M10×1.5 → 10 − 1.5 = 8.5 mm drill.

For blind holes, drill the hole at least 3–4 thread pitches deeper than the required thread depth to allow for the tap chamfer. A common mistake on the shop floor is drilling just to the thread depth, which causes tap breakage on the last few turns.

Common Metric Tap Drill Sizes

Tip: For stainless steel or hard materials, use a slightly larger tap drill (e.g. major dia − 0.9×pitch) to reduce tap breakage risk. The slight reduction in thread engagement is worth the reliability on the shop floor.