One more question what am I measuring? ie when it says 3mm x 4mm which number is diameter and which is length and does the length include the screw head or not? Sorry I am ignorant on this stuff. Just an old lady vaper trying to sort this all out.
Screw and Bolts are often described by what is call a simple "Callout".
When you see a Metric Screw for say a Post Screw, it might have a simple Called Out of M3mm x 4mm or M3 x 4mm
The "M" in M3mm or just M3 tells you it is a Metric Thread. And the "3" tells you it is 3mm in Diameter.
If you Measure the Diameter of the Screw Threads, you will find that it is Actual a Smidge less than 3mm. This Smidge Less is for Clarence. Because it you try to put a exactly 3mm Threaded Screw in a exactly 3mm Thread Hole, it would be Very Hard to turn the Screw. If not Impossible
The 4mm in this M3mm x 4mm or just M3 x 4mm tells you what the Length of the Threads is measured from the End of the Screw to under the Head of the Screw. The Size of the Head isn't considered. But Many Times in a Detailed Specification they will give the LOA (Length Over All) which is the Lenght of the Threads + the Length of the Head combined.
The "True" Callout would also include the Thread Pitch. Which would be the Distance between any two Threads. But since Many Metric Thread are so Common as coming in Only One Pitch, which is considered "Standard", then Pitch will sometimes by omitted. And seller with just use a Simple Callout.
Here's a Vid that probably does a Better Job of Explaining what I just said...