Fluorescence Correlation Spectroscopy (hereafter called FCS) is a technique used to determine the diffusion coefficient D by following the number of fluorescent molecules contained in a given spot at various times. We derive an expression, relating the variance of the estimated D to several experimental parameters. The main results give a qualitative description of how the statistical uncertainty of this inherently stochastic technique depends on adjustable variables, such as duration of the experiment, maximal autocorrelation lag, beam size, etc.