A method is provided for measuring edge exclusion on a workpiece that includes a wafer having a film disposed thereon. The method is performed by a CMP system employing a platen and a thickness sensor coupled to the platen and positioned to repeatedly travel a path over the edge of the film during polishing. The method comprises measuring the thickness of the workpiece during selected iterations of the probe path, and establishing from the wafer thickness measurements the length of time the probe is over the film (t.sub.on) during the selected iterations. Edge exclusion is determined for at least one iteration utilizing a function related to t.sub.on.