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?
What do the numbers in screw sizes M3, M4 etc. mean
screwsterminology
Related Solutions
"6" is a #6-size screw. Screw gauges are a measure of the head size and shaft size, and are roughly linear but not quite a 1:1 relationship (a #8 screw is a little less than twice the diameter of a #4 screw). There isn't a good system for converting gauge to a calibrated measurement, so you're best off consulting a table like this: http://hingedummy.info/screwinfopage2.htm
Screws have three basic measurements: gauge, threads per inch, and shaft length in inches. So, you may also see a measurement like 6-32 x 1 1/2". This means it's a #6 diameter, with 32 threads per inch (almost double the normal thread count as a standard wood screw) and an inch and a half long. When the middle number is absent (6 x 1 1/2"), the screw has the "normal" number of threads per inch for that size and type of screw (for #6 wood screws that's 18).
M5 Pan head machine screw.
Edit: M5 screws have a 5mm shaft.
Edit2.1 (my apologies): This is an M5-0.80 screw. 0.8 refers to the thread pitch, which (for metric screws) is measured by dividing 9.6mm by 12 threads, which equals 0.80. The picture above shows a minor optical illusion of 12 threads over 10mm (pitch = 0.8333); but M5-0.85 screws simply don't exist.
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Best Answer
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:
Thread charts are easily found if you need to know about more sizes.