Use the following steps to determine the correct bracelet length. When designing, keep in mind that the diameter of the beads you string has a significant effect on the interior circumference of the finished bracelet. By allowing for this you will get a perfect fit every time.
Measure the customer's wrist, leaving some slack for a comfortable fit.
Subtract the length of the clasp assembly. The result is the desired bead length for your bracelet.
Allow for the diameter of your beads by simply adding 3 beads (when using rounds). For other shapes of beads, triple the diameter of the bead and add enough beads to equal that number. For example, a 3mm diameter x 5mm length tube bead, means you'll add 2 beads to your design. 3 (mm of diameter) x 3 (to triple the diameter) = 9. In other words, two 5mm beads will add 10mm to your bracelet length, compensating for the diameter of the beads with 1mm of space.
If you are unable to measure the wrist of the intended recipient of the bracelet, a great option is to use an extender chain. This option enables the wearer to adjust the bracelet for a customized fit.