How to measure screws

screws

I was just looking at some timber screws online and was confused about the sizing. For example, the dimensions of one screw were, "18 – 7 x 200mm". Another one was 14g x 150mm.
From what I understand, the first number in both cases refers to the diameter, 7 to the number of threads per inch/mm (not given in the second example), and then the length.
Are the units used for the diameter the same in both examples (i.e, could the "18" in the first example also be written 18g)?

Here are links to product pages for each of the screws:

  1. 18 – 7 x 200mm: https://www.bunnings.co.nz/buildex-18-7-x-200mm-landscaping-construction-screw_p2410356
  2. 14g x 150mm: https://www.bunnings.co.nz/zenith-14g-x-150mm-galvanised-type-17-bugle-head-timber-batten-screws-25-pack_p2410108

Best Answer

You are correct in assuming that 18 is actually 18G (gauge), according to Buildex packaging. Typically the second number is TPI (threads per inch) but with metric dimensions the second number is thread pitch (distance between threads). In this case 7mm seems unlikely, so I will assume TPI since I am unable to find documentation stating otherwise. 200mm is the overall length of the screw.

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