Metric bolts and screws are labelled with an M followed by their diameter in millimeters — an M6 bolt is 6 mm across the threads. If you work with inch tools or are trying to match a part from another country, the names alone are not much help. This guide converts the most common sizes, M3 to M12, into inches and explains thread pitch, wrench sizes and drill sizes so you pick the right tool the first time.
How Metric Bolt Sizes Are Named
A full metric bolt description looks like M8 × 1.25 × 30:
- M8 — nominal thread diameter: 8 mm (0.315″).
- 1.25 — thread pitch: the distance in millimeters from one thread to the next. If it is left out, the standard coarse pitch is assumed.
- 30 — length in millimeters, measured under the head (about 1.18″).
Inch bolts work differently: they are described by diameter and threads per inch (TPI), for example 1/4″-20. To compare the two, convert pitch to TPI with TPI = 25.4 ÷ pitch.
Metric Bolt Sizes in Inches (M3–M12)
| Size | Diameter | Diameter in inches | Coarse pitch | ≈ Threads per inch | Closest inch bolt |
|---|---|---|---|---|---|
| M3 | 3 mm | 0.1181″ | 0.5 mm | 50.8 | #4 (0.112″) |
| M4 | 4 mm | 0.1575″ | 0.7 mm | 36.3 | #8 (0.164″) |
| M5 | 5 mm | 0.1969″ | 0.8 mm | 31.7 | #10 (0.190″) |
| M6 | 6 mm | 0.2362″ | 1 mm | 25.4 | 1/4″ (0.250″) |
| M8 | 8 mm | 0.3150″ | 1.25 mm | 20.3 | 5/16″ (0.3125″) |
| M10 | 10 mm | 0.3937″ | 1.5 mm | 16.9 | 3/8″ (0.375″) |
| M12 | 12 mm | 0.4724″ | 1.75 mm | 14.5 | 1/2″ (0.500″) |
Important: the “closest inch bolt” is only the nearest size — the threads are not compatible. An M6 × 1.0 bolt will not properly thread into a 1/4″-20 nut even though the diameters look similar (6.00 mm vs 6.35 mm). Forcing mixed threads damages both parts.
Wrench, Socket and Hex Key Sizes
The size of spanner or socket you need is the distance across the flats of the hex head, not the thread diameter. Current ISO standards and the older DIN standard differ for a few sizes, which is why you may find both 16 mm and 17 mm heads on M10 bolts.
| Size | Hex nut / bolt head (ISO) | Older DIN size | Socket cap screw hex key | In inches (spanner) |
|---|---|---|---|---|
| M3 | 5.5 mm | same | 2.5 mm | 0.217″ |
| M4 | 7 mm | same | 3 mm | 0.276″ |
| M5 | 8 mm | same | 4 mm | 0.315″ |
| M6 | 10 mm | same | 5 mm | 0.394″ |
| M8 | 13 mm | same | 6 mm | 0.512″ |
| M10 | 16 mm | 17 mm | 8 mm | 0.630″ |
| M12 | 18 mm | 19 mm | 10 mm | 0.709″ |
Metric heads should always be turned with metric tools. A 13 mm head is 0.512″ across — a 1/2″ (0.500″) socket is too small, and a 9/16″ (0.5625″) socket is loose enough to round the corners.
Tap Drill and Clearance Hole Sizes
For coarse metric threads, the tap drill size is simply diameter minus pitch (for example M8: 8 − 1.25 = 6.75, rounded to a 6.8 mm drill). A clearance hole is slightly larger than the bolt so it passes through freely.
| Size | Tap drill (coarse) | Clearance hole (medium fit) |
|---|---|---|
| M3 | 2.5 mm (0.098″) | 3.4 mm (0.134″) |
| M4 | 3.3 mm (0.130″) | 4.5 mm (0.177″) |
| M5 | 4.2 mm (0.165″) | 5.5 mm (0.217″) |
| M6 | 5.0 mm (0.197″) | 6.6 mm (0.260″) |
| M8 | 6.8 mm (0.268″) | 9 mm (0.354″) |
| M10 | 8.5 mm (0.335″) | 11 mm (0.433″) |
| M12 | 10.2 mm (0.402″) | 13.5 mm (0.531″) |
See our drill bit size chart if you only have fractional inch drills available.
How to Measure an Unknown Bolt
- Measure the thread diameter with calipers across the outside of the threads. A reading close to a whole millimeter (5.9–6.0 mm, for example) suggests a metric bolt; a reading near 6.35 mm (0.25″) suggests 1/4″.
- Check the pitch with a thread gauge, or measure the length of 10 threads and divide by 10. 12.5 mm over 10 threads = 1.25 mm pitch (M8 coarse).
- Look at the head: metric bolts usually have property class markings such as 8.8 or 10.9; inch bolts use radial lines.
A good pair of digital calipers that reads in both mm and inches makes this much easier.
Frequently Asked Questions
What is M6 in inches?
An M6 bolt is 6 mm in diameter, which is 0.2362″. The closest inch size is 1/4″ (0.250″), but the threads are not interchangeable.
What size wrench fits an M8 bolt?
13 mm under both ISO and DIN standards. That is 0.512″ across the flats.
What does 8.8 mean on a bolt head?
It is the property class (strength grade). 8.8 is a common medium-strength steel bolt; 10.9 and 12.9 are higher strength.
Related: Small metric sizes in inches · Drill bit size chart · How to read a tape measure