A bracelet and a bangle are sized differently
A bracelet opens, so it is sized from the wrist plus an allowance for how loosely it should sit. A bangle does not open, so the wrist is irrelevant — the only question is whether it will pass over the knuckles of a folded hand. Sizing a bangle from the wrist guarantees it will not go on, and it is far and away the commonest bangle-buying mistake.
For a bracelet, measure around the bare wrist just below the wrist bone, snug but not tight. Add about 1.3 cm for a snug fit that stays put, 2 cm for a normal fit that moves a little without sliding over the hand, or 2.5 cm for a deliberately loose drape. For a bangle, tuck your thumb into your palm as if putting one on, measure around the widest part of the folded hand, and divide by pi to get the minimum inner diameter.
What changes the number
Width matters. A bracelet over about 15 mm across, or a cuff with real thickness, sits higher off the wrist and consumes length — add a few millimetres beyond the standard allowance. A heavy pendant or charm cluster does the same by pulling the chain down.
If you fall between the lengths shops stock, size up rather than down. Links can be taken out of a bracelet that is slightly long; a short one cannot be extended without adding a section that will show.