Metric bolts are specified by a simple code, but that code carries a lot of information. Understanding what M6, M12 or M36 really means — and the thread pitch behind each — prevents costly ordering mistakes. This guide explains the metric sizing system and provides a reference chart from M6 to M36 for buyers and workshops.
How Metric Bolt Sizes Are Named
A metric bolt designation like M12 x 1.75 x 60 breaks down as follows:
- M12 — the nominal outside diameter of the thread is 12 mm.
- 1.75 — the thread pitch, meaning the distance between adjacent threads is 1.75 mm.
- 60 — the bolt length in millimetres (usually measured under the head for standard hex bolts).
If no pitch is stated, the bolt is assumed to use the standard coarse pitch for that diameter.
Metric Bolt Size Chart (Coarse Thread)
| Size | Diameter (mm) | Coarse Pitch (mm) | Wrench / Hex (mm) |
|---|---|---|---|
| M6 | 6.0 | 1.00 | 10 |
| M8 | 8.0 | 1.25 | 13 |
| M10 | 10.0 | 1.50 | 16 or 17 |
| M12 | 12.0 | 1.75 | 18 or 19 |
| M16 | 16.0 | 2.00 | 24 |
| M20 | 20.0 | 2.50 | 30 |
| M24 | 24.0 | 3.00 | 36 |
| M30 | 30.0 | 3.50 | 46 |
| M36 | 36.0 | 4.00 | 55 |
Note: wrench sizes can vary slightly between DIN 933/931 and ISO 4017/4014 standards, so confirm the standard when ordering.
Coarse vs Fine Thread Pitch
Most metric bolts use coarse thread by default, but fine-pitch versions exist. Fine threads (for example M12 x 1.25 instead of M12 x 1.75) have more threads per length, giving:
- Higher tensile strength at the thread root
- Better resistance to loosening under vibration
- Finer torque adjustment
Coarse threads, however, install faster, tolerate minor damage and dirt better, and are the standard choice for general construction and machinery.
Common Ordering Mistakes to Avoid
Buyers often mix up length measurement conventions — countersunk bolts are measured overall, while hex bolts are measured under the head. Another frequent error is assuming pitch: an M12 coarse (1.75) nut will not thread onto an M12 fine (1.25) bolt. Always state diameter, pitch, length, grade, and finish together on your purchase order.
Getting the Right Fasteners
When in doubt, send a sample or drawing to your supplier. A source factory can match thread pitch, grade and coating exactly and produce non-standard lengths on request. Accurate specification up front saves rework and shipping delays later.
About Chaoshuo Trading
Chaoshuo Trading is a source fastener factory in China, offering factory-direct pricing and full OEM/ODM customization on bolts, nuts, screws, washers and anchors. From standard DIN/ISO/ANSI parts to custom drawings, we manufacture to your spec with in-house quality control and export-ready packaging. Get a fast quote today:
- Email: info@chaoshuotrade.com
- WhatsApp: +86 158 0311 9164
- Website: www.chaoshuotrade.com