M12 thread sizes
10.2 mm tapping drill · 13.5 mm clearance
1.75 mm coarse pitch · 1.5 mm fine · 10 mm hex key · 19/18 mm spanner
What M12 is used for
Heavy machinery mounts and structural steelwork.
Every number for M12
| Major diameter | 12 mm |
| Coarse pitch | 1.75 mm |
| Fine pitch | 1.5 mm (M12×1.5) |
| Tapping drill, coarse | 10.2 mm (12 − 1.75 = 10.25) |
| Tapping drill, fine | 10.5 mm |
| Clearance hole, normal | 13.5 mm |
| Hex key (socket head) | 10 mm |
| Spanner (hex head) | 19 mm — or 18 mm under ISO 4017 |
| Tapped depth, steel | 18.0 mm (1.5 × diameter) |
| Tapped depth, aluminium | 24.0 mm (2 × diameter) |
Where the drill size comes from
A tapping drill leaves enough material for the tap to cut a thread, and the amount it has to leave is one pitch: drill = major diameter − pitch. For M12 coarse that is 12 − 1.75 = 10.25 mm, and since 10.25 mm is not a drill anyone sells, the charts round to 10.2. The rule holds for the fine series too — M12×1.5 is drilled 10.5 mm.
Going slightly larger gives a shallower thread that is easier to tap and weaker; going smaller risks breaking the tap. The standard drill aims at about 75% thread engagement, which is where the strength stops improving enough to be worth the extra effort.
Nearby sizes
| Thread | Pitch | Tapping drill | Clearance | Hex key |
|---|---|---|---|---|
| M4 | 0.7 mm | 3.3 mm | 4.5 mm | 3 mm |
| M5 | 0.8 mm | 4.2 mm | 5.5 mm | 4 mm |
| M6 | 1 mm | 5 mm | 6.6 mm | 5 mm |
| M8 | 1.25 mm | 6.8 mm | 9 mm | 6 mm |
| M10 | 1.5 mm | 8.5 mm | 11 mm | 8 mm |
| M14 | 2 mm | 12 mm | 15.5 mm | 12 mm |
Frequently asked questions
What drill size for M12?
10.2 mm for a coarse thread. The rule is major diameter minus pitch: 12 − 1.75 = 10.25, and 10.2 mm is the nearest drill actually sold.
What is the clearance hole for M12?
13.5 mm for a normal fit under ISO 273. A close fit is about 13.1 mm and a loose fit around 14.5 mm — the clearance hole is the one in the part the bolt passes through, not the one being tapped.
What size spanner for an M12 bolt?
19 mm or 18 mm — the standards disagree at this size. DIN 933 specifies 19 mm across the flats and ISO 4017 specifies 18. Both bolts are M12; only the head differs, which is why a socket set can seem to be missing a size.
What is the pitch of M12?
1.75 mm coarse, or 1.5 mm in the fine series (M12×1.5). Pitch is the distance between adjacent thread crests, so one full turn advances the bolt 1.75 mm.