Determine on which wrist you prefer to wear your bangle, and make a
fist with that hand.
Where your fingers join the back of your fisted hand, locate the apexes of
the knuckles of your little (5th) finger and index finger.
With a cloth tape measure, measure between these apexes. This is your bangle size.
Look the picture below
size minus 3mm or 1/8" then
The result is your bangle size of ID (inside diameter)