Metric Thread Pitch Chart & How to Measure Metric Bolt Lengths

When selecting metric fasteners, understanding thread pitch and bolt length is essential to ensure compatibility and proper fitment.

This guide explains the standard coarse thread pitches for common metric bolt sizes and how metric bolt lengths are measured.

What is Thread Pitch?

Thread pitch is the distance between adjacent threads, measured in millimetres (mm).

For example, an M10 x 1.5 bolt has:

• M10 = 10mm nominal thread diameter
• 1.5 = 1.5mm distance between each thread

The larger the pitch number, the coarser the thread.

Standard Metric Coarse Thread Pitch Chart

These are the standard ISO coarse thread pitches commonly used across construction, engineering, manufacturing, automotive, and general fastening applications.

How to Measure Metric Bolt Length

Metric bolt lengths are generally measured from the underside of the bolt head to the end of the bolt.

For example:

• An M10 x 50 bolt has a 10mm thread diameter and a length of 50mm.
• The 50mm measurement is taken from underneath the head to the end of the threaded shank.

Hex Head Bolts

For hex head bolts, flange bolts, and most externally driven fasteners:

Measure from:

• The underside of the head
• To the end of the bolt

The head itself is not included in the length measurement.

Example:

• M12 x 75 Hex Bolt = 75mm measured from under the head to the tip.

Countersunk Bolts and Screws

Countersunk fasteners are measured differently.

Because the head sits flush with the surface after installation, the overall length includes the head.

Measure from:

• The top of the countersunk head
• To the end of the bolt

Example:

• M8 x 40 Countersunk Bolt = 40mm total length including the head.

Coarse vs Fine Threads

Coarse threads are the most common metric thread type and offer:

• Faster installation
• Better resistance to thread damage
• Improved performance in softer materials
• Easier assembly in dirty or outdoor environments

Fine threads have a smaller pitch and are often used in specialised engineering, automotive, and high-vibration applications.

A commonly used ISO rule for partially threaded hex bolts is:

• For bolts up to 125 mm long:
  • Thread Length = (2 × Diameter) + 6 mm
• For bolts longer than 125 mm:
  • Thread Length = (2 × Diameter) + 12 mm

Examples

Please note that this is not 100% accurate for the thread lengths of bolts but it is the most common way to find the thread lengths.

Need Help Choosing the Right Fastener?

If you're unsure which thread size, pitch, or length you need, contact the Bolt Direct team. We're happy to help identify the correct fastener for your application.