In this paper the optical properties of spherical GNPs in aqueous solutions are analyzed, both theoretically and experimentally, for diameters from 3 to 120 nm. Methods to calculate the particle diameter (d) from UV-vis spectra are derived, and analytical relations between the extinction efficiency (Qext) and d are established, which allow the determination of the particle concentration (c). The major aim is to provide researchers preparing gold hydrosols or using them with a simple and fast method to determine size and concentration. An extension of the calculations described here for alternative materials and other dielectric environments is possible, and the reader is encouraged to develop his own calibration curves using our FORTRAN 77 program, which can be found in the Supporting Information together with several tables which allow the reader to determine the size and the concentration of GNPs in aqueous solutions from UV-vis spectra without performing calculations.