The distance constant, which is a true instrument constant, can in principle be determined in a wind tunnel by recording, at a particular wind speed, the time it takes to be for the rotor to obtain equilibrium with the wind after starting from zero rotation rate. It seems easier to use the method applied here, namely the power-spectrum method. A field experiment where the output from a sonic anemometer and a cup anemometer at the height of about 2 m and laterally displaced by approximately 2 m measured simultaneously the wind velocity with high temporal resolution. The duration of each of the 19 runs varied between 5.5 h and 37 h. The wind velocity in these periods varied from 1.76 m/s to 8.64 m/s. The result was that the distance constant was measured to be 1.81 m with the standard deviation 0.04 m.