Mechanical vibration of a rotating test body is translated into a periodic signal which represents a combined vector of the unbalance means of the rotating body and the mechanism that rotates it. The periodic signal is divided into two orthogonal vector components. The magnitude of the two vector components are converted into digital signals and applied to a computer. Unknown values of four constants are determined by a preliminary test having three successive stages. In the first and second stages the test body is rotated to register the digital signals; in the second state the angular position of the test body is displaced a predetermined amount with respect to its first stage position. In the third stage of the test, a trial dead weight of a known mass is mounted on the test body and rotated to register the digital signals. The computer operates on the registered digital signals to determine the four constants in accordance with a set of equations. After the preliminary test, the data registered in the first stage of the preliminary test is substituted into another set of equations using the four constants to determine the dynamic unblance mass and its angular location.