Skip to content
HobbyHQ
Search

Home machining

Metric thread and tapping drill reference

The standard ISO metric coarse table — pitch, tapping drill and clearance drill — with the engagement arithmetic shown beside it as a check rather than as the answer.

The question this answers: “What drill do I use before tapping M6, and what size hole does an M6 bolt pass through?”

Around 65–75% is normal. More sharply increases tapping torque for very little extra strength.

Result

Tapping drill (standard)
5 mm
About 92% thread engagement — what a standard drill index gives you.
Drill for 70% engagement
5.24 mm
The arithmetic. Use the nearest drill you actually own.
Clearance drill
6.6 mm
For the part the fastener passes through rather than threads into.
Pitch
1 mm
M6 coarse. Fine pitches exist for most of these and take different drills.

Worth knowing about this answer

  • A reference table, not a machining recommendation. It gives no speeds, no feeds and no tapping torque, because those depend on the material, the machine and the tap rather than on the thread.
  • The listed tapping drill is the standard commercial size. The computed figure beside it is the arithmetic for a chosen engagement, and where they differ the standard size is what a drill index actually contains.
Show the working
  1. ThreadM6 coarse6 mm × 1 mm pitch
  2. Drill for chosen engagement6 − (70% × 1 × 1.0825)5.24 mm
  3. Standard tapping drillFrom the standard table5 mm

Tapping drill (standard): 5 mm. Drill for 70% engagement: 5.24 mm. Clearance drill: 6.6 mm. Pitch: 1 mm

How it works

A lookup of the standard ISO metric coarse series, which is where the tapping and clearance sizes come from. Alongside it, the conventional engagement formula — nominal diameter minus the chosen percentage of pitch multiplied by 1.0825 — is shown so the standard figure can be sanity-checked rather than taken on trust. The two differ slightly because commercial tapping drills sit a little above 100% thread to reduce tapping torque.

What it assumes

  • ISO metric coarse pitches. Fine pitches exist for most of these sizes and take different drills.
  • The 1.0825 constant is the standard geometric factor relating thread depth to pitch for a 60-degree thread form.

Where it stops being reliable

  • A reference table, not machining guidance. It gives no speeds, no feeds, no tapping torque and no lubricant recommendation, because those depend on the material, the machine and the tap rather than on the thread — the same reason the home machining page publishes no cutting parameters.
  • It cannot tell you whether a thread is strong enough for a load. Engagement length matters more than drill size for that, and it is a design question.
  • Imperial, BSP, BSW and BA threads are not covered.

Runs entirely in your browser — nothing you type is sent anywhere. Last reviewed 2026-08-17. More on Home machining.