Tap Drill Basics

Why tap drill size matters

The hole must be smaller than the bolt’s major diameter so the tap can cut material and form threads. Too small and you risk breaking the tap or excessive torque; too large and thread engagement is weak. Standard practice targets about 75% thread depth for a balance of strength and tap life.

Metric tap drill (rule of thumb)

Tap drill diameter ≈ major diameter − pitch. Example: M6 x 1.0 → 6 − 1 = 5.0 mm. Coarse and fine pitches give different drill sizes for the same nominal diameter.

Tap Drill Calculator · Tap Drill Chart

Drill and tap sequence

Drill the hole to the tap drill size, then tap with the correct tap (hand or machine). Use cutting fluid for metals. For through holes, a through tap is fine; for blind holes use a plug or bottoming tap as needed.

FAQ

What is tap drill size?

Tap drill size is the diameter of the hole you drill before tapping. It is smaller than the bolt major diameter so that the tap can form threads; typically chosen for about 75% thread depth.

How do you calculate tap drill size for metric?

A common rule is: tap drill diameter = major diameter minus pitch. For M6 x 1.0, drill = 6 − 1 = 5 mm. Use our Tap Drill Calculator for exact values.

What if I drill the hole too large?

Oversized holes give shallow threads and weak engagement. Use the recommended tap drill size from a chart or our calculator for reliable threads.