The threaded rod diameter refers to the diameter of the external thread processing surface. The thread diameter is strictly called: the nominal diameter of the thread. For example, the nominal diameter of the external thread M20 is 20, but its actual value is generally about 0.2 smaller than the nominal diameter. In other words, the nominal diameter of the external thread M20 is 20, and the rod diameter is φ19.8. There is no "rod diameter" for internal threads.