Tool for measuring of the diameter of a body with circular cross section, where two arms (1) are provided in a fixed angle to another, in that a distance sensor device (2) is provided axially between the two arms (1), in that the distance sensor device (2) is adapted to register a contact point for the distance sensor device (2) against the body with respect to the contact points of the arms against the body, and that a viewing device (3) is adapted to calculate and show the diameter of the circle on the basis of the three contact points on the circle.