Various thread gauges with complex geometry are used for thread control in industry. Over time, as a result of long-term use, they wear out and they need to be calibrated periodically. The most important dimension of a thread, its pitch diameter, is usually measured by the contact method using 1D measuring devices. The European Association of National Metrology institutes EURAMET lists different calibration categories in its guide EURAMET cg-10. In order to be able to determine the pitch diameter according to categories 2a, 2b and 3, it is necessary to measure thread pitch. In this thesis the thread and the thread gauges for internal thread are described. Also, in the theoretical part of the thesis, a brief description of the metrological infrastructure is given, and methods of thread pitch measurement are described, with an emphasis on optical systems. In the experimental part of the thesis, thread pitch is measured in the laboratory for precise length measurements at the Faculty of Mechanical Engineering and Naval Architecture using a tool microscope and a universal 1D measuring device, and the results are compared.