How to measure the screws to match how stores label them

When I buy screws, the container often has measurement labels, e.g. : 50/4 or 20/2.5 and such. I'm pretty sure the first number is a length in millimeters, and the second number is a width in millimeters; and there's definitely a correlation, i.e. the 50/4's are longer than 40/4's or 40/5's, and the /4's are narrower than /5's and so on.

However, when I try to actually measure these screws – it doesn't quite fit. My 50/4 screws are around 43 or 44 mm long, including the head; and without the head it's about 40mm. Their width – well, with the threading, and at the widest point, I suppose you could argue it's 4, though I kind of have to squint to make it happen…

So, my question is: Given a screw, how am I supposed to measure its diameter and length so that the values end up matching how stores label it?

Notes:

• I live in Palestine/Israel, which is a mostly-metric country (although some remnants of Imperial measurements remain.)
• I realize the answer might be country-specific; but it can't be store-specific, since this "larger number/smaller number" scheme is ubiquitous in hardware stores here.