Watch band or watch strap size is determined by the width of the band end that attaches to the watch. If you already have a leather band for the watch in question, you can check the back to see if it has a size imprint, shown in millimeters. Otherwise, you will need to find the lug width (distance between the inside of the lugs on the watch head). You can easily measure it yourself in one of the below three ways.
Rounding: Rule of thumb for a replacement watch band is that it’s better for the watch band to be tight rather then loose. If you need to round your measurement, rounding up is usually your best bet. A leather, rubber or nylon watch strap can be slightly flexible to fit.
Watch Band Conversion Chart (Inches to Millimeters):