M10 x 1.25 tap drill size
Drill 8.75 mm for an M10 x 1.25 fine thread: the 10 mm major diameter minus the 1.25 mm pitch, giving about 77% thread. In practice an 8.8 mm drill is used (74% thread).
| Drill | mm | inch | Thread % | Typical use |
|---|---|---|---|---|
| 8.6 mm | 8.6 | 0.3386 | 86.2% | Tight: high tapping torque, tap breakage risk |
| 8.7 mm | 8.7 | 0.3425 | 80.1% | General purpose |
| 11/32" (inch drill) | 8.73 | 0.3438 | 78.1% | General purpose |
| 8.75 mm (ISO 2306 nominal) | 8.75 | 0.3445 | 77.0% | General purpose |
| 8.8 mm | 8.8 | 0.3465 | 73.9% | General purpose |
| 8.9 mm | 8.9 | 0.3504 | 67.7% | Hard or tough material, deep or blind holes |
| 9.0 mm | 9.0 | 0.3543 | 61.6% | Hard or tough material, deep or blind holes |
| Thread | Tap drill | Tap drill (in) | Clearance (ISO 273 medium) |
|---|---|---|---|
| M8 x 1.0 | 7.0 mm | 0.2756 in | 9.0 mm |
| M10 x 1.25 (this page) | 8.75 mm | 0.3445 in | 11.0 mm |
| M10 x 1.0 | 9.0 mm | 0.3543 in | 11.0 mm |
| M12 x 1.5 | 10.5 mm | 0.4134 in | 13.5 mm |
| M12 x 1.25 | 10.75 mm | 0.4232 in | 13.5 mm |
Practical notes
For through holes, a spiral-point (gun) tap pushes chips ahead and suits machine tapping; for blind holes use a spiral-flute tap and drill a few pitches deeper than the full thread you need.
Typical engagement lengths: about 1 × diameter (10 mm) in steel, 1.5 × in cast iron or brass and 2 × in aluminium. Going from about 75% to full thread adds little strength but much more tapping torque.
Thread-forming (roll) taps need a larger hole, about 9.38 mm here (D − 0.5 × P). Treat that as a starting point and check the tap maker's chart; forming suits ductile materials such as aluminium, mild steel and brass, not cast iron.
Fine pitches like M10 x 1.25 are used for adjustment screws, thin-walled parts and where resistance to loosening under vibration matters. The shallower thread makes drill size more critical: a 0.1 mm change shifts engagement by about 6 percentage points.
Clearance holes come from ISO 273: fine (close) 10.5 mm, medium (normal) 11.0 mm, coarse (loose) 12.0 mm. Use fine for location, medium as the default and coarse when hole positions vary.
FAQ
What size drill for an M10 x 1.25 tap?
Use an 8.75 mm drill (0.3445 in) for a cut M10 x 1.25 tap, per ISO 2306 (major diameter minus pitch). It gives about 77% thread. An 8.8 mm drill is the common practical choice (74% thread).
What clearance hole for an M10 bolt?
ISO 273 gives 10.5 mm (fine/close), 11.0 mm (medium/normal) and 12.0 mm (coarse/loose) for M10, whatever the pitch. Medium is the usual default.
What inch drill can I use for an M10 x 1.25 tap?
The closest inch drill is 11/32" (0.3438 in, 8.73 mm), which gives about 78% thread on M10 x 1.25.
How is M10 x 1.25 different from other M10 pitches?
Same 10 mm major diameter, different pitch, so a different tap drill: M10 x 1.5 (8.5 mm drill), M10 x 1.0 (9.0 mm drill). The taps and bolts are not interchangeable with M10 x 1.25 (8.75 mm drill).
Sources: ISO 261 — ISO general purpose metric screw threads, general plan (pitches) · ISO 724 — basic dimensions (minor diameter D1 = D − 1.0825 P) · ISO 2306 — drills for use prior to tapping (tap drill = D − P) · ISO 273 — clearance holes for bolts and screws · Machinery's Handbook — percentage-of-thread formula
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