M12 x 1.25 tap drill size
Drill 10.75 mm for an M12 x 1.25 fine thread: the 12 mm major diameter minus the 1.25 mm pitch, giving about 77% thread. In practice a 10.8 mm drill is used (74% thread).
| Drill | mm | inch | Thread % | Typical use |
|---|---|---|---|---|
| 10.6 mm | 10.6 | 0.4173 | 86.2% | Tight: high tapping torque, tap breakage risk |
| 10.7 mm | 10.7 | 0.4213 | 80.1% | General purpose |
| 27/64" (inch drill) | 10.72 | 0.4219 | 79.1% | General purpose |
| 10.75 mm (ISO 2306 nominal) | 10.75 | 0.4232 | 77.0% | General purpose |
| 10.8 mm | 10.8 | 0.4252 | 73.9% | General purpose |
| 10.9 mm | 10.9 | 0.4291 | 67.7% | Hard or tough material, deep or blind holes |
| 11.0 mm | 11.0 | 0.4331 | 61.6% | Hard or tough material, deep or blind holes |
| Thread | Tap drill | Tap drill (in) | Clearance (ISO 273 medium) |
|---|---|---|---|
| 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 (this page) | 10.75 mm | 0.4232 in | 13.5 mm |
| M14 x 1.5 | 12.5 mm | 0.4921 in | 15.5 mm |
| M16 x 1.5 | 14.5 mm | 0.5709 in | 17.5 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 (12 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 11.38 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 M12 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) 13.0 mm, medium (normal) 13.5 mm, coarse (loose) 14.5 mm. Use fine for location, medium as the default and coarse when hole positions vary.
FAQ
What size drill for an M12 x 1.25 tap?
Use a 10.75 mm drill (0.4232 in) for a cut M12 x 1.25 tap, per ISO 2306 (major diameter minus pitch). It gives about 77% thread. A 10.8 mm drill is the common practical choice (74% thread).
What clearance hole for an M12 bolt?
ISO 273 gives 13.0 mm (fine/close), 13.5 mm (medium/normal) and 14.5 mm (coarse/loose) for M12, whatever the pitch. Medium is the usual default.
What inch drill can I use for an M12 x 1.25 tap?
The closest inch drill is 27/64" (0.4219 in, 10.72 mm), which gives about 79% thread on M12 x 1.25.
How is M12 x 1.25 different from other M12 pitches?
Same 12 mm major diameter, different pitch, so a different tap drill: M12 x 1.75 (10.25 mm drill), M12 x 1.5 (10.5 mm drill). The taps and bolts are not interchangeable with M12 x 1.25 (10.75 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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