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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.
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  • Just be happy you're trying to figure out screws. Remind me again how big a 6d nail is? Yeah, that's "6 penny". Thank the British for that one. At least you know that a 6d nail is smaller than a 16d nail... :) – FreeMan Jun 30 '20 at 0:12
  • @FreeMan we moved on from “odd” units but if you kept them that’s not our decision. – Solar Mike Jun 30 '20 at 7:21
  • Actually, @SolarMike, we seem to have mostly moved on from that, too. Most boxes of nails I've seen lately (I usually buy screws these days), are labeled in inches - 3 1/2" x 0.131" is a common framing nail size that is pretty close to the old money "16d". – FreeMan Jun 30 '20 at 10:44
  • Inches are still part of the bizarro British-Imperial world AFAIC :-P – einpoklum Jun 30 '20 at 10:58
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How do I measure my screws?, let me count the ways.

Buy yourself a gauge.

For not much money you can take the guess work out of it.

Stainlesstown Nut Bolt Thread Gauge

SAE and Metric Bolt Gauge Check a Thread Fastener Tool

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  • Well, for the diameter, ok, that works. But for the length - I don't see how that answers the question... also - that second guage costs over 100 USD (!) – einpoklum Jun 29 '20 at 23:13
  • @einpoklum There is a length gauge on the first example i linked to. I Included just two examples of many. You are free to do research on your own for more affordable options ( I have faith in you ) or i can do some for a fee. It might not be the answer you would like but it is an answer. – Alaska Man Jun 29 '20 at 23:25
  • I realize there's a length gauge, but the problem is what exactly I need to measure - between what points. – einpoklum Jun 30 '20 at 6:28
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The first number is usually the "major" diameter, meaning the diameter of the outside peaks of the threads. The second value is usually the length, but that would NOT be inclusive of the head, so it is from the INSIDE surface of the head to the end. A 4mm screw that is 50mm long would be designated as an M4x50. These are ISO international standards for metric fastener designations, but obviously you are not referring to 50mm diameter screws (they would be bolts at that point) that are only 4mm long. So all I can think of is that your store has some sort of unique numbering system that is not used elsewhere. In other words you would have to ask them...

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  • 1. You're thinking about the US perhaps? 2. It's not a single store, it's everywhere here; I'll make it clearer. – einpoklum Jun 30 '20 at 6:25
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The only correct way to specify screws is by these three parameters:

  • Length
  • Nominal diameter (the hole they freely go through, also the diameter of their unthreaded shank)
  • Thread pitch

All units are in millimeters. Note that thread pitch is the number of millimeters per thread (which is the inverse of the Imperial method of threads per unit distance, i.e. inch).

Stated as M diameter - thread pitch x length

Take an M6-1.00 x 50mm. Fits a 6mm hole, 1.00 thread pitch, and 50mm long.

That is the only way you should be specifying fasteners. If a store wants to do it in some wackadoodle way for their own benefit, well, that’s no different than having a SKU number or a UPC bar code number. Pay it no attention; it does not matter.

Now, as far as measuring length, that is all about how far it goes into the material. If it has a countersunk (cone shaped) head so the head is designed to be flush with the surface, then you measure from the top of the head. Any sort of button or hex or other surface mount head, you measure from the shoulder (bottom of head). So a x 40 bolt with a 3mm tall head will be 43mm in total.

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