There are a number of variables to consider when working out the final length of a watch strap once it is fitted to your watch:
With a one piece watch strap such as the Military Watch Straps
the length of the strap does not change when fitted, however with two piece straps as shown below, the size of the watch head must be added to the length
of the buckle and strap ends to calculate the total length of the strap.
Take the length of the watch strap from the details of the strap you want to buy, or from the table below, and add your watch length to it.
Take off 20mm to allow for the strap to be buckled on the 3rd to last hole and leave enough 'tail' to be secured by the strap loops,
and this is the final maximum strap length and the circumference of the maximum wrist it will fit.
The minimum wrist size the strap fits is usually 30mm less than the maximum wrist size.
Watch Strap Lengths (excluding buckle)
Standard length straps = 170mm for straps up to 14mm width.
Standard length straps = 190mm for straps 16mm width and above.
Extra-long straps = 190mm for straps up to 14mm width.
Extra-long straps = 210mm for straps 16mm width and above.
Extra-extra-long straps = 215mm for straps up to 14mm width.
Extra-extra-long straps = 225mm for straps 16mm width and above.