To determine which size cylinder(s) you need, you must first take the measurements. You measure the outside size from the locking screw (of the cylinder) (1) to the outside of the fitting (2). A cylinder should never protrude more than 3 mm on the outside of the door fitting. Note this size (outside).
You also do this for the inside (3), but the cylinder may protrude from the fitting if desired. If you want a cylinder for a Nuki Smart lock, you must add at least 2.5 mm to mount the Nuki smart lock on the cylinder.
With a half cylinder, you only measure one side.
With this one you only measure the inside.