Abstract: A thickness measuring gauge for insulating glass which includes, in the preferred embodiment, a sighting member with a front sight, a rear sight and a sight tube through which the user takes an angled sighting of the insulating glass with the device in contact with the near side surface thereof. A separate reflective member is situated on the far side of the insulating glass to reflect into view the image of a target member. The target member is attached to a movable slide affixed to the device, which may have its position adjusted so as to align the target reflection with the sight. A series of steps are formed on the movable slide, each of which represent a different thickness of single glass sheet (two or more of which comprise the insulating glass), while marks inscribed on the base of the device each represent a standard thickness of insulating glass.