Cut a strip of paper a centimetre wide, wrap it around the wrist just below the bone, snug but not tight, and mark where it overlaps. Laid flat against a ruler, that mark is the figure to choose. A tape measure does the same work in one step.
Measure at the end of the day rather than the start, and on the arm the bracelet will be worn on — the two are rarely the same. If the reading falls between two figures, take the larger: the clasp is set to the measure, and a bracelet that turns is easier to live with than one that binds.
A photograph of the marked strip beside a ruler settles any doubt, and the bench will ask for one if the reading needs it.