The formation of functional nanoscopic domains is an inherent property of plasma membranes. Stimulated emission depletion combined with fluorescence correlation spectroscopy (STED-FCS) has been previously used to identify such domains; however, the information obtained by STED-FCS has been limited to the presence of such domains while crucial parameters have not been accessible, such as size (), the fraction of occupied membrane surface (), in-membrane lipid diffusion inside () and outside () the nanodomains as well as their self-diffusion (). Here, we introduce a quantitative approach based on a revised interpretation of the diffusion law. By analyzing experimentally recorded STED-FCS diffusion law plots using a comprehensive library of simulated diffusion law plots, we extract these five parameters from STED-FCS data. That approach is verified on ganglioside nanodomains in giant unilamellar vesicles, validating the Saffman-Delbrück assumption for . STED-FCS data in both plasma membranes of living PtK2 cells and giant plasma membrane vesicles are examined, and a quantitative framework for molecular diffusion modes in biological membranes is presented.