In this article optimal supersaturation for cooling and evaporation crystallization are presented. This information is based on a simplified model which describes the metastable zone width of activated nucleation. The model assumes that the nucleation step is very fast and the occurrence of the shower of nuclei which is used to define the metastable zone width is caused by the growth of activated nuclei. In future new sensors for the measurement of supersaturation will help to operate batch crystallizers at the optimal supersaturation during the entire batch time.