Abstract This is the third paper presented by the author to the A.S.M.E. on the flow of air in fan ducts. In the first paper, it was established that the normal flow in such ducts is vortical in character and that the use of straighteners is necessary in order to obtain a reliable air-volume measurement. The second paper contained proposals for (1) the adoption of a specified design of pitot tube as a standard in fan testing and (2) the use of square-edged inlet or discharge orifices for measuring air volumes except at or near maximum fan capacities. In the present paper, the results of an extended series of investigations on a simplified form of square-edged orifice are presented. The discharge coefficients obtained in these investigations are identical with those found in extensive German tests and adopted in the standard rules of the V.D.I. and by the International Standards Association. This agreement justifies the use of the coefficients for the large-sized ducts and the varieties of velocity distribution found in fan discharge ducts. The substitution of inlet and discharge orifice measurements for pitot-tube traverses and the adoption of these coefficients in a fan-testing code would, the author believes, greatly shorten and simplify the process of testing a fan and would make it more accurate.