The std. deviation (SD) in fluorescence correlation spectroscopy (FCS) has been mostly neglected in applications. However, the knowledge of the correct SD is necessary for an accurate data evaluation, esp. when fitting theor. models to exptl. data. In this work, an algorithm is presented that considers the essential features of FCS. It allows prediction of the performance of FCS measurements in various cases, which is important for finding optimal exptl. conditions. The program calcs. the SD of the exptl. autocorrelation function online. This procedure leads to improved parameter estn., compared to currently used theor. approxns. for the SD. Three methods for the calcn. of the SD are presented and compared to earlier anal. solns., calcn. directly from fluorescence intensity values, by averaging several FCS measurements, or by dividing one measurement into a set of shorter data packages. Although the averaging over several measurements yields accurate ests. for the SD, the other two methods are considerably less time consuming, can be run online, and yield comparable results. [on SciFinder (R)]