threadsizes

M10 x 1.25 Tap Drill Size

ISO metric fine · 1.25 mm pitch · ISO 261 / 965-1, class 6H

What size drill bit for a M10 x 1.25 tap?

8.8 mmmetric drill
0.3465inch
8.80mm
74%thread
No 8.8 mm drill? Use 11/32" (78%) or S (72%).

To scale section of the tapped hole

Section of a M10 x 1.25 tapped hole to scale: 8.8 mm (0.3465) drill, thread length 1.5 times the diameter. ⌀10 mm P 1.25 mm DRILL 8.8 mm Thread 1.5 × D +5 P drill
Drawn from the M10 x 1.25 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 M10 x 1.25
DrillInchmmThread6H
8.7 mm 0.3425 8.70 80% ✓
11/32" 0.3438 8.73 78% ✓
8.8 mmUse 0.3465 8.80 74% ✓
S 0.3480 8.84 72% ✓
8.9 mm 0.3504 8.90 68% ✓
9 mm 0.3543 9.00 62% ✗
T 0.3580 9.09 56% ✗
9.1 mm 0.3583 9.10 55% ✗

By material Aluminum, brass, mild steel: 8.8 mm (74%) · Stainless, tool steel: 8.9 mm (68%)

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 (8.65 mm to 8.91 mm).

M10 x 1.25 ISO 261, 965-1, class 6H

Major diameter10 mm / 0.3937 in
Pitch1.25 mm
Pitch diameter (basic)9.188 mm
Minor diameter 6H8.647–8.912 mm
Thread-forming tap drill9.4 mm 0.3701 in

Clearance hole ISO 273

Close10.5 mm10.5 mm
Normal11 mm11 mm
Loose12 mm12 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

  • 10 mmthread length for full strength in steel (1 × D)
  • 15 mmin cast iron or brass (1.5 × D), up to 20 mm in aluminum
  • +6.25 mmextra drill depth below the thread for a plug tap (5 pitches), +2.5 mm with a bottoming tap

Shop notes

M10 x 1.25 sits between the coarse 1.5 mm pitch and the 1 mm extra fine, and the 8.8 mm drill gives about 74% thread. It has one of the friendlier windows among the M10s: everything from 8.7 mm (80%) to 8.9 mm (68%) passes the 6H minor diameter check.

The inch shop does well here. An 11/32 (0.3438 in, 78%) and a letter S (0.3480 in, 72%) both pass, and the 11/32 is a common drill. Note the irony: the same 11/32 is too big for the coarse M10 x 1.5, where it falls outside the limits. A drill that is right for one M10 is not automatically right for the others.

The trap on this page is the 9 mm drill, which is correct for M10 x 1. In an M10 x 1.25 hole it leaves 62% thread and fails the minor diameter check, as does letter T.

In hard material, lean toward the 8.9 mm rather than going bigger. It keeps you inside the limits with less tap torque, and the step after it is already a weak thread.