How to Size a Watch Strap
1.Determine what type of strap the watch requires. You will need to determine both the material it is made out of (leather, nylon, rubber/silicon, etc.), and, in the case of metal watch band whether it is a stretch band or closes with a clasp.
2.Determine the width of the strap you will need. If you are unsure of the strap width that will fit your watch, try measuring the area where the strap will connect to the watch. The width will probably be somewhere between 8 and 30 mm.
3.Measure your wrist size by wrapping the measuring tape around the area of your wrist where your watch would normally be. Write down your wrist's circumference.
4.Measure the length of the watch head (lug to lug) and subtract the length of the watch head from your wrist size to determine what length of band you will need to fit snugly on your wrist. For example, if your wrist is 7 inches in circumference and your watch head is 1 inch across, you are left with 6 inches of wrist strap*.
*6 inches of wrist strap - this is the length of the strap when buckled.
Many leather straps are adjustable what means that longer pieces of the straps have holes. In result there will be different length when buckled to the first hole and when buckled to the last hole. For example, if the strap's max length (when buckled to the first hole) is 8 inches and the strap's min length (when buckled to the last hole) is 5 inches, the strap is correct for a wrist 7 inches in circumference and a watch head 1 inch across.
Wrist size: 7 inches
Watch Strap Length:
The required length of the strap (6 inches) is between 8 and 5 inches so the strap is correct. If the required length is above 8 inches the strap will be too short. If the required length is below 5 inches the strap will be too long.