M3 thread sizes
2.5 mm tapping drill · 3.4 mm clearance
0.5 mm coarse pitch · 2.5 mm hex key · 5.5 mm spanner
What M3 is used for
The smallest size in general workshop use — enclosures, brackets, 3D-printed parts.
Every number for M3
| Major diameter | 3 mm |
| Coarse pitch | 0.5 mm |
| Tapping drill, coarse | 2.5 mm (3 − 0.5 = 2.5) |
| Clearance hole, normal | 3.4 mm |
| Hex key (socket head) | 2.5 mm |
| Spanner (hex head) | 5.5 mm |
| Tapped depth, steel | 4.5 mm (1.5 × diameter) |
| Tapped depth, aluminium | 6.0 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 M3 coarse that is 3 − 0.5 = 2.5 mm. The rule holds for the fine series too — subtract whichever pitch you are cutting.
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 |
|---|---|---|---|---|
| M2 | 0.4 mm | 1.6 mm | 2.4 mm | 1.5 mm |
| M2.5 | 0.45 mm | 2.05 mm | 2.9 mm | 2 mm |
| 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 |
Frequently asked questions
What drill size for M3?
2.5 mm for a coarse thread. The rule is major diameter minus pitch: 3 − 0.5 = 2.5.
What is the clearance hole for M3?
3.4 mm for a normal fit under ISO 273. A close fit is about 3.2 mm and a loose fit around 3.8 mm — the clearance hole is the one in the part the bolt passes through, not the one being tapped.
What size spanner for an M3 bolt?
5.5 mm across the flats for a standard hex head. A socket head cap screw of the same size takes a 2.5 mm hex key instead.
What is the pitch of M3?
0.5 mm coarse. Pitch is the distance between adjacent thread crests, so one full turn advances the bolt 0.5 mm.