Abstract: A diameter measuring instrument that comprises a straight-edge and two legs or jaws, wherewith the distance between the legs can be read-off. The invention is characterized in that the straightedge (2) is provided with a leg (3) which is fixed in relation to the straightedge and a leg (4) which is movable rela-tive to the straightedge, or alternatively with two legs that are movable relative to the straightedge; in that each leg (3, 4) includes a laser (5, 6) which is designed to emit a visible laser beam (7, 8) in a direction that coincides with the length direction of respective legs (3, 4); and in that the laser beams (7, 8) are parallel to one another.