Abstract: An on-line system and method provides for monitoring the build up of yarn package diameter which detects winding defects, including missing or gained filaments, in multifilamentary yarn spinning processes. The system and method derive and compare yarn package denier for several yarn packages wound on a common winding apparatus. Diameter ranges are used to divide a yarn package into various measurement zones. Denier measurements are derived for each zone from two on-line, real-time inputs: yarn package speed and time to traverse the specified zone. Comparison of denier for diverse packages being simultaneously wound on one winding apparatus are compare to each other and determined to be within a specified tolerance. The apparatus operator is notified of out of tolerance measurements so that remedial action may be taken. The system also provides for constant and periodic updating of the denier measurement algorithm to account for process variations.