How to Read Metric Screw Sizes: M3, M4, M5 & M6 Explained
Labels such as M3×8, M4×12 and M6×20 tell you much more than a random stock code. The first number identifies the nominal metric thread diameter and the second number normally.
Labels such as M3×8, M4×12 and M6×20 tell you much more than a random stock code. The first number identifies the nominal metric thread diameter and the second number normally identifies the fastener length in millimetres. When thread pitch is included, the format may appear as M6×1.0×20, meaning a 6 mm nominal thread, 1.0 mm pitch and 20 mm length.
Reading the size correctly matters because an M4 screw will not properly fit an M3 thread, and two fasteners with the same nominal diameter can still differ if one uses a fine pitch. The head shape does not change the M-size; it changes the way the screw bears on the part and how the length is measured.
Quick answer: Labels such as M3×8, M4×12 and M6×20 tell you much more than a random stock code.
What to know about how to read metric screw sizes
The M designation
The letter M identifies an ISO metric screw thread. The number immediately after it is the nominal outside diameter of the thread in millimetres, so M3 means a nominal 3 mm thread and M6 means a nominal 6 mm thread.
The length number
The second number normally gives fastener length in millimetres. On socket head, pan head and hex-head fasteners, length is generally referenced from under the head. Countersunk heads are commonly measured overall because the head enters the countersink.
Thread pitch
Pitch is the axial distance from one thread crest to the next. Coarse pitch is often assumed when no pitch is stated, but precision parts, imported assemblies and fine-thread applications should always be checked rather than assumed.
Head and drive are separate attributes
An M3×10 fastener could be Phillips pan head, Torx countersunk, Allen countersunk or another style. Size alone does not tell you the head profile, drive recess, material or strength.
How to choose the right fastener
- Confirm the mating thread: Check the tapped hole or nut before ordering. A thread gauge or known-good mating fastener is better than visual estimation.
- Measure the length correctly: Identify whether the head is included in the stated length. Countersunk fasteners deserve special attention.
- Check pitch when the assembly is critical: If the fastener came from machinery, automotive equipment or an imported assembly, verify pitch explicitly.
- Check clearance around the head: A socket head may require more head height than a countersunk screw; a hex bolt needs spanner clearance around the head.
- Record the full specification: For repeat purchasing, note diameter, pitch, length, head, drive, material and finish instead of writing only M4×12.
Quick comparison
| Marking | Meaning | Example |
|---|---|---|
| M3×8 | 3 mm nominal thread, 8 mm length | Small electronics and compact assemblies |
| M4×12 | 4 mm nominal thread, 12 mm length | Brackets, enclosures and machinery |
| M6×20 | 6 mm nominal thread, 20 mm length | General mechanical joints |
| M6×1.0×20 | 6 mm thread, 1.0 mm pitch, 20 mm length | Fully specified metric thread |
A practical ordering workflow
Before placing an order for how to read metric screw sizes, turn the application into a short specification. Start with the mating thread and physical envelope, then confirm the head or drive, material and finish. If the fastener is part of a loaded, vibrating, outdoor or frequently serviced joint, add the relevant mechanical, corrosion and maintenance requirements before choosing a SKU.
For repeat purchases, record the exact approved description in your bill of materials or purchasing system instead of relying on a photograph or an old sample. That record should include the dimensions that control fit, the material or property class, the finish, and any standard or certificate requirement. This makes replacement orders easier to compare and helps prevent accidental substitutions.
When a dimension, torque value, strength requirement or corrosion condition is safety-critical, use the applicable engineering drawing, current standard and manufacturer documentation as the final authority. Catalogue guidance is useful for narrowing choices, but it should not override the requirements of the actual joint.
How to verify the specification before ordering
Make the first verification on the actual assembly or approved drawing. For confirm the mating thread, check the tapped hole or nut before ordering. A thread gauge or known-good mating fastener is better than visual estimation. For measure the length correctly, identify whether the head is included in the stated length. Countersunk fasteners deserve special attention. Do not assume that an older fastener removed from the machine is still the approved specification; previous repairs may have introduced substitutions.
If the part is being introduced into production, buy a small validation quantity before committing to a large lot. Check thread engagement by hand, seating of the head or nut, tool clearance, surface marking, interference with nearby parts and the condition of the mating thread after assembly. Where traceability or mechanical properties matter, compare received markings and documents with the purchase specification. This small verification step is especially useful when changing suppliers, materials, finishes or dimensional standards.
Common mistakes to avoid
- Measuring the head diameter and assuming that is the M-size.
- Measuring a countersunk screw from under the head instead of overall.
- Assuming a screw is coarse pitch without checking the mating thread.
- Ordering only by M-size and forgetting head style or drive type.
- Using approximate inch measurements to identify a metric thread.
Recommended products and categories
You can continue from this guide by viewing M4×12 Allen socket head cap screw, M6×20 hex bolt and metric screws, nuts and bolts.
Technical references
The following independent technical references are useful for checking terminology, standards and engineering background. They are provided for reference and do not imply endorsement of any product sold on this site.
Frequently asked questions
What does M3 mean on a screw?
M3 denotes a metric thread with a nominal major diameter of 3 mm.
Does M4 mean the screw is exactly 4 mm across?
It refers to the nominal thread diameter. Actual manufactured dimensions and tolerances are controlled by the applicable thread standard.
What does M3×10 mean?
It normally means an M3 metric thread and a 10 mm fastener length. The head style determines where that length is measured from.
Do I always need to specify thread pitch?
Not always for common coarse-thread stock, but you should specify it whenever there is a possibility of fine pitch, special thread form or a critical mating part.
Related reading
Final buying checklist
- Confirm the nominal thread diameter and thread pitch.
- Confirm the required length and the correct head-to-tip measuring method.
- Check head style, drive style and installation-tool access.
- Match material and finish to the service environment and mating materials.
- Check strength, locking, documentation and application limits where they matter.
Treat the size marking as the start of the specification, not the whole specification. A repeatable order description should include thread size, pitch when needed, length, head, drive, material and finish.
Ready to select a part? Browse our Screws, Nuts & Bolts catalogue and filter by thread size, material, finish, head style and drive type.