A bangle does not go on over the wrist. It goes on over the knuckle at the base of the thumb, which is the widest fixed point on the hand and the number that actually decides the fit.
How to measure
Bring your thumb across to the base of your little finger, as if putting on a glove. Wrap a strip of paper around the widest part of that shape, mark it, lay it flat and measure in millimetres. Divide by 3.14. That is your internal diameter.
Add nothing for comfort. A bangle that clears the hand easily will spin and eventually meet a table edge. The correct size is the one that requires a little pressure and a little soap.
The part nobody mentions
Hands swell in heat and in the evening. Measure in the morning, in a cool room, and expect a snug bangle to be genuinely difficult to remove in August. That is normal, and it is why bangles are usually left on.