A.      Determine on which wrist you prefer to wear your bangle, and make a fist with that hand.

B.      Where your fingers join the back of your fisted hand, locate the apexes of the knuckles of your little (5^th) finger and index finger.

C.      With a cloth tape measure, measure between these apexes.  This is your bangle size.

Look the picture below

This size minus 3mm or 1/8" then
The result is your bangle size of ID (inside diameter)