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Computer simulation provides a means of determining the relative effects of various factors on spray drift while field experiments to measure spray drift have the limitation that many variables cannot be controlled. A Windows Version computer program (DRIFTSIM) was developed to rapidly estimate the mean drift distances of water droplets discharged from atomizers on field sprayers. This program interpolates values from a large data base of drift distances originally calculated for single droplets with a flow simulation program (FLUENT). The simulations of drift distances up to 200 m (656 ft) included temperatures (10-30 C; 50-86F), discharge heights (0-2.0 m; 0-6.56 ft), initial downward droplet velocities (0-50 m/s; 0-164 ft/s), relative humidity (10-100%), wind velocities (0-10.0 m/s; 0-32.8 ft/s), droplet sizes (10-2000 m), droplet size distribution in Dv.1, Dv.5 and Dv.9, and 20% turbulence intensity. Variables can be either in metric or English units. For the input of droplet size distribution, drift distances are reported along with portion of volume in each class such as provided by many droplet size analyzers. The accuracy of the program FLUENT was verified with a uniform droplet generator and wind tunnel. The program indicates the relative effects of the input variables on drift distances and should, especially for large droplets, provide reasonable accuracy for many field applications.