What do the numbers in screw sizes M3, M4 etc. mean?

Question

I noticed that in some (many?) countries, screw sizes are listed as "M+number", e.g. M3, M4, M5 and so on.

What does this size designation mean?

Answer 1

There are a few more numbers for a complete description. It's pretty much a world-wide system described by several ISO standards, but there are occasional oddities (slight differences between ISO and DIN standard sizes on a few items for instance)

You asked about M3, M4, etc - that describes (only) the outside diameter of the screw - or the size of an unthreaded hole the screw would slip through.

A full description is like M6-1X25 Which describes the outer diameter (6) , the distance from one thread to the next (1) (pitch) and the length (25) (all in millimeters.)

An M6 could also be an M6-0.75 (fine thread pitch) X whatever length. When not specified, the coarse thread is generally assumed, but sometimes the assumption is wrong...

Coarse and Fine pitch for the sizes you asked about:

• M3-0.50 and M3-0.35
• M4-0.7 and M4-0.5
• M5-0.8 and M5-0.5

Thread charts are easily found if you need to know about more sizes.

Answer 2

Mn for a number n means that the diameter of the screw, projected onto the transverse plane, is n milimeters - including the threading.

The "M" stands for "Metric" (as opposed to British-imperial units).

Illustration:

Remember the Mn designation does not tell you what the length of the screw (along its axis) is.