Measuring internal diameters is a common metrological task in real production conditions. Consequently, every measurement result, including the result of measuring the inner diameter, must be expressed in the form of complete metrological information. Complete metrological information consists of measurement result and measurement uncertainty, with coverage factor and probability level information. Therefore, in this final work, measurements of internal diameters were performed on five control rings using three different manual measuring instruments (vernier calliper, micrometer for internal measurements and three-point micrometer). The measurements were performed in the National Laboratory for Length located at the Faculty of Mechanical Engineering and Naval Architecture in Zagreb. Before the very beginning of the experimental part, the individual measuring instruments used in practice for measuring internal diameters and their main characteristics and the ways in which they are used were studied in detail, which is explained and described in detail later in this paper. In the experimental part of the paper, the characteristics of the used manual measuring instruments and the used control rings are listed. After the measurement results were achieved and the measurement uncertainties were estimated, the results were compared with the reference value, ie the value specified in the valid Certificate of Calibration of the Measurement Ring. Based on the results of the analysis conducted in the conclusion of the paper, the importance of presenting complete metrological information is discussed.