threadsizes

M12 Tap Drill Size (M12 x 1.75)

ISO metric coarse · 1.75 mm pitch · ISO 261 / 965-1, class 6H

What size drill bit for a M12 tap?

10.2 mmmetric drill
0.4016inch
10.20mm
79%thread
No 10.2 mm drill? Use 13/32" (74%) or Y (77%).

To scale section of the tapped hole

Section of a M12 x 1.75 tapped hole to scale: 10.2 mm (0.4016) drill, thread length 1.5 times the diameter. ⌀12 mm P 1.75 mm DRILL 10.2 mm Thread 1.5 × D +5 P drill
Drawn from the M12 x 1.75 basic profile. Hatched area is the part; the thread runs 1.5 × D and the drill goes 5 pitches deeper for a plug tap.

Drill options % of thread

Drill options for M12 x 1.75
DrillInchmmThread6H
10.1 mm 0.3976 10.10 84% ✗
10.2 mmUse 0.4016 10.20 79% ✓
Y 0.4040 10.26 77% ✓
10.3 mm 0.4055 10.30 75% ✓
13/32" 0.4063 10.32 74% ✓
10.4 mm 0.4095 10.40 70% ✓
Z 0.4130 10.49 66% ✗
10.5 mm 0.4134 10.50 66% ✗
10.6 mm 0.4173 10.60 62% ✗

By material Aluminum, brass, mild steel: 10.2 mm (79%) · Stainless, tool steel: 10.4 mm (70%)

75% (orange tick) is the usual target. Hard or gummy metals like stainless are often tapped at 60 to 65% to spare the tap; aluminum, brass and plastic take 75% or a little more. The last column checks the hole against the 6H minor diameter limits (10.11 mm to 10.44 mm).

M12 x 1.75 ISO 261, 965-1, class 6H

Major diameter12 mm / 0.4724 in
Pitch1.75 mm
Pitch diameter (basic)10.863 mm
Minor diameter 6H10.106–10.441 mm
Thread-forming tap drill11.2 mm 0.4409 in

Clearance hole ISO 273

Close13 mm13 mm
Normal13.5 mm13.5 mm
Loose14.5 mm14.5 mm

For a screw that passes through the part. Normal fit is the default; close fit locates the part more accurately but leaves no room for hole position error.

Hole depth rule of thumb

  • 12 mmthread length for full strength in steel (1 × D)
  • 18 mmin cast iron or brass (1.5 × D), up to 24 mm in aluminum
  • +8.75 mmextra drill depth below the thread for a plug tap (5 pitches), +3.5 mm with a bottoming tap

Shop notes

The standard 10.2 mm drill gives about 79% thread. That is on the high side, and it is also the smallest drill that passes the 6H minor diameter check: 10.1 mm and letter X are both too tight. The diameter minus pitch rule lands between 10.2 mm and 10.3 mm, and in tough steel the 10.3 mm (75%) is the more comfortable of the two.

In inch drills, letter Y (0.4040 in, 77%) and 13/32 (0.4062 in, 74%) are both good, and the 13/32 is often easier to find. That gives a range from 10.2 mm to 10.4 mm (70%) where nothing goes wrong. Past that, letter Z and 10.5 mm drop to 66% and leave the limits.

Two drills that do not belong here

The 10.5 mm is the correct drill for M12 x 1.5, and it sits in the same drawer. In a coarse M12 hole it is outside the limits. The 27/64 is worse: it is the tap drill for 1/2-13, and here it leaves about 57% thread.

That 1/2-13 is also the usual mix-up. It is bigger (0.500 in against 0.4724 in) with a coarser pitch (1.9538 mm against 1.75 mm), so a 1/2 bolt will not go into an M12 hole, and an M12 in a 1/2-13 hole has too little thread to trust.