The cun was part of a larger system, and represented one-tenth of a chi ("Chinese foot"). In time the lengths were standardized, although to different values in different jurisdictions. (See chi (unit) for details).
In Japan, the corresponding unit, sun(寸(すん),sun?), was standardized at 1000⁄33 mm (~3.030 cm, ~1.193 in, or ~0.09942 ft).
A section of an old Hong Kong ruler, showing the last (10th) cun of a chi. One can see that the chi in that jurisdiction was exactly equal to 14 and 5/8 of an inch. A metric ruler is shown next to it for comparison