Watch Strap measurement information
When choosing a replacement watch strap it is important to get the correct size or else the strap will look inappropriate for the watch. To correctly measure what size strap is required, the dimension needed is the distance between the shoulders of the watch itself, into which the spring bars fit. As indicated by the image below.
Clasp measurement information
A clasp (also referred to as a butterfly clasp and deployment clasp) is used to replace the buckle on a watch strap. To correctly measure what size clasp is required, the dimension needed is the distance at the end of the watch strap where the buckle is attached. This measurement will also be the width of the buckle at the point of attachment. This is indicated by the image below.