DIY Doctor UK DIY Forums

I've just learned of the different screw size types and that one number 8 isn't the same as another number 8. You conversion chart [url=https://www.diydoctor.org.uk/projects/screwsize.htm]here[/url] shows that, for example, a 4 x 40mm is equal to 8 x 1½".

However, the gauge diameters in both imperial and metric measurements aren't measured in respective units (or are they)?

Your table implies that the metric gauge is a measurement in mm as 'mm' is shown in the column. Perhaps it should say 'metric gauge' instead.

The gauge is an imperial term and relates to the diameter of the screw, but is not a measurement of the diameter per se. It is the first number quoted, the second being the length in inches.

In the metric sizing system for screws, the measurements are quoted in mm, diameter of the thread first, then length second. The diameter is measured across the outer edge of the thread and is often called the major diameter.

We have update the more detailed description here (a couple of times over the years), if you need more: https://www.diydoctor.org.uk/projects/screwsize.htm