An investigation of wind tunnel blockage effects on cup anemometer calibrations has been performed. A slender anemometer body in combination with three different cup anemometer rotors of different sizes have been calibrated in five different wind tunnels. Four wind tunnels were of the closed-test-section type and one wind tunnel of the open jet type. One of the closed tunnels had a significantly larger test section than the others and was used as a reference. The smallest rotor was so tiny that blockage effects were negligible in the tunnels used. The large rotor was sufficiently large for blockage effects to be significant. The results were not the expected. The blockage effects in the three closed test sections were initially assumed to be of similar magnitude and follow a An investigation of wind tunnel blockage effects on cup anemometer calibrations has been performed. A slender anemometer body in combination with three different cup anemometer rotors of different sizes have been calibrated in five different wind tunnels. Four wind tunnels were of the closed-test-section type and one wind tunnel of the open jet type. One of the closed tunnels had a significantly larger test section than the others and was used as a reference. The smallest rotor was so tiny that blockage effects were negligible in the tunnels used. The large rotor was sufficiently large for blockage effects to be significant. The results were not the expected. The blockage effects in the three closed test sections were initially assumed to be of similar magnitude and follow a common trend. The blockage effects in the open test section were expected to be lower in magnitude and of opposite sign. The unexpected results could be explained by the test conditions. Due to budgetary limitations in the project it was not possible to use other set-ups for measuring the reference wind speed than the standard set-up. Significant influences, from the test object on the measurement of the tunnel wind speed, could therefore not be excluded. Hence, no general conclusions about blockage could be drawn. Provided that the frequency values from each tunnel were reliable, then individual correction recommendations for each tunnel could be given: The method to calibrate anemometers of different sizes and normalize the results is straight forward and easy to use and in spite of the fact that no general conclusions about blockage effects could be drawn it is a belief that the method works well provided that more efforts are put on the localization of the position to measure the undisturbed WT wind speed.