The fluorescent particles used in FCS are small and thus experience thermal motions in solution. The simplest FCS experiment is thus normal 3D diffusion, for which the autocorrelation is:
where  is the ratio of axial to radial  radii of the measurement volume, and  is the characteristic residence time. This form was derived assuming a Gaussian measurement volume. Typically, the fit would have three free parameters—G(0), , and —from which the diffusion coefficient and fluorophore concentration can be obtained.
With the normalization used in the previous section, G(0) gives the mean number of diffusers in the volume <N>, or equivalently&mdash;with knowledge of the observation volume size&mdash;the mean concentration:
where the effective volume is found from integrating the Gaussian form of the measurement volume and is given by:
: D gives the diffusion coefficient: