Take a bangle which fits perfectly (neither loose nor too tight).
Take a string and place it along the center of the bangle (as denoted by the two arrow heads in the above illustrations) with the string taut.
Take a standard measurement scale and measure the portion of string which was inside the bangle (again keep the string taut while measuring). This is your bangle diameter.
Choose the bangle size that matches your measurement with the diameter measurements above. If there is no perfect match go for the immediate larger measure.
