Measuring The Watch Strap (Lug) Width
The lug width is the distance between the case lugs (where the strap attaches to your watch) as highlighted by the blue arrow in the illustration.
To measure the width, you should use either a ruler or a set of digital callipers to measure the gap. The distance between the lugs is the width of the watch strap you will be wanting to purchase.
Note that watch strap widths are generally provided in mm.