Sprays with narrowly distributed drop sizes can be produced by atomization systems operating according to the principle of breakup of stretched liquid threads. The mean drop size depending on the process parameters is of major interest for a reliable process design. Thus, a model for the description of the breakup process of stretched liquid threads with laminar flow is introduced. Thread extension by gravity and gas-induced disturbances are considered in order to predict the mean droplet size. The numerical and experimental results exhibit satisfying accordance.