Ways to measure your wrist:
Use a tape measure, wrap the it around the wrist on which you plan to wear your bracelet. Make a note of the number at the point where the tape meets the 0. To ensure the bracelet fits comfortably, add 1/2 to 3/4 of an inch. You can then add or subtract according to whether you prefer a loose, exact or tight fit.
Using an existing bracelet:
Find a bracelet that fits you correctly. Set it down flat. Measure with a ruler the length in centimeters from the top of the clasp to end of the bracelet. Make a note of the measurement. Order your bracelet in a size as close as possible to this measurement.
String and Ruler
If you are unable to find a measuring tape, simply find a piece of string or a strip of paper and wrap it around your wrist at the position where you'd like to wear your bracelet. Make note of where the string/strip overlaps and lay it flat over a ruler to get the measurement.
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