The cylindrical ruler is an elongated, hollow, cylindrical tube of known diameter having an axial slit extending the length of the tube. Indicia defining a circumferential scale are disposed around one end of the tube for measuring the length of an arc. Indicia defining a ruled scale extend lengthwise down the tube adjacent the slit for measuring length and width. Since the tube has a known diameter, it may be used to measure the inner or outer diameter of a pipe, the outer diameter or radius of curvature of a cylindrical lens, etc. The tube may also be used as a drawing tool to draw arcs, circles, or rectilinear lines on a cylindrical object. Preferably, the cylindrical ruler is furnished as a plurality of cylindrical tubes of progressively increasing diameter that can telescope within each other for storage and transport in a protective case.