What size watch strap is required?
The easiest way to determine the strap size for your watch is to measure the distance between the two lugs. This can be achieved with a ruler or a measuring device as shown in the example. Measure distance between the lugs (the area where your strap fits into the case). Strap size measurements are made in millimetres (mm).
As you can see by the below image the Vintage Omega Seamaster will fit a 18mm Watch strap.