Accurate hydraulic design of laterals is one of the most important factors for ensuring water distribution uniformity in drip irrigation systems. In order to keep water distribution uniformity at an appropriate value, pressure and flow rate variations of emitters along a lateral should be within acceptable limits. This makes the determination of the optimum lateral lengths for drip irrigation systems necessary. The objective of this study was to develop a simplified empirical model able to predict the optimum lateral length to obtain uniform water distribution. The model developed is valid for a 16 mm nominal pipe diameter, with cylindrical co-extruded emitters and emitter spacing ranging from 0.2 m to 1 m. Optimum lateral length data was obtained from 26 different drip irrigation pipes. The model developed accounted for 98.3% of the variation in the optimum lateral lengths. The model was obtained by considering pipes with inside diameters ranging from 12.3 to 14.2 mm, emitter inside diameters ranging from 11.5 to 12.6 mm, emitter lengths ranging from 31.6 to 68.2 mm, emitter spacing ranging between 0.2 and 1.0 m, emitter discharges ranging from 1.75 to 4.301 h(-1), and emitter flow variations from 10% to 20%, with gradients extending downhill or uphill <3%. The errors in the optimum lateral lengths increased with 3% downhill gradients for 0.75 and 1.0 m emitter spacing. The prediction model can be used to calculate the optimum lateral lengths without using any software. (C) 2015 IAgrE. Published by Elsevier Ltd. All rights reserved.