A tool for forming substantially rectangular cross sections on conduits. Two mateable collars are disposable about the exterior of a conduit and have forming surfaces which can be pressed against the conduit's wall to predetermined extents to cause the conduit to assume the desired outside rectangular dimensions. Such pressing is provided by two threaded fasteners, each of which extend through one of the collar members and into threaded engagement with the mateable collar. The assembled collar configuration may be disengaged from, rotated about, and reclamped on the exterior of the conduit so as to provide uniform wall formation if the shape of the desired conduit permits such collar rotation. To assure complete contact between the conduit's exterior and the forming surfaces of the collar, two wedge members are axially inserted in the conduit's opening.