To calculate the length required use a tape measure around the part of your wrist where you are most comfortable wearing your bracelet.  On average you will need to add an extra 3cm to the length.  This will allow enough spare area to fit you snugly when the bracelet is completely full.  If you like to wear your bracelets loose then add 4cm to your wrist size or otherwise if you require less charms for your completed bracelet then reduce the amount between 1-3cm pro-rata