M2.5 thread sizes
2.05 mm tapping drill · 2.9 mm clearance
0.45 mm coarse pitch · 2 mm hex key · 5 mm spanner
What M2.5 is used for
Laptop and small appliance assembly.
Every number for M2.5
| Major diameter | 2.5 mm |
| Coarse pitch | 0.45 mm |
| Tapping drill, coarse | 2.05 mm (2.5 − 0.45 = 2.05) |
| Clearance hole, normal | 2.9 mm |
| Hex key (socket head) | 2 mm |
| Spanner (hex head) | 5 mm |
| Tapped depth, steel | 3.8 mm (1.5 × diameter) |
| Tapped depth, aluminium | 5.0 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 M2.5 coarse that is 2.5 − 0.45 = 2.05 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 |
| M3 | 0.5 mm | 2.5 mm | 3.4 mm | 2.5 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 |
Frequently asked questions
What drill size for M2.5?
2.05 mm for a coarse thread. The rule is major diameter minus pitch: 2.5 − 0.45 = 2.05.
What is the clearance hole for M2.5?
2.9 mm for a normal fit under ISO 273. A close fit is about 2.7 mm and a loose fit around 3.3 mm — the clearance hole is the one in the part the bolt passes through, not the one being tapped.
What size spanner for an M2.5 bolt?
5 mm across the flats for a standard hex head. A socket head cap screw of the same size takes a 2 mm hex key instead.
What is the pitch of M2.5?
0.45 mm coarse. Pitch is the distance between adjacent thread crests, so one full turn advances the bolt 0.45 mm.