Determine the correct size of the bracelet
The bandwidth is based on the the lug size of the watch case and is therefore predetermined. Here measurements are taken from inside to inside.
Measure the wrist:
With the help of a tailor's tape measure the wrist size can be measured very easily.
Alternatively, cut an approx. 2 cm long strip from a sheet of paper, wrap it around your wrist, cut it off at the end and measure this length.
Measure watch case:
Now measure the case of the watch from lug to lug (ideally from spring bar hole to spring bar hole).
Height of the bracelet:
The height of the bracelet and the height of the case should also match.
Determine bracelet length:
Subtract the case dimensions from the wrist size. This is the approximate size of the bracelet you will need. Possibly add a few millimeters, as the strap is never worn too tight and the folding clasp on the back also needs some space.
Here is an example calculation:
Let's suppose your wrist size is approximately 170 mm and the case of the watch is 42 mm in diameter. However, the size of the case from one hole for the spring bars to the other is relevant. So this would be more like 45 mm. If this size is substract of the wrist size, we would have 125 mm left.
Usually a little space is preferred between the bracelet and the wrist. Hence, a 130 mm bracelet would fit well. Especially since the specified strap sizes for a metal bracelet are always approximate, as you also have to consider the clasp on the back of the wrist under the strap.
Please also note that some straps can be adjusted in the folding clasp or by removing or adding extension links.