A relatively easier and more accurate method for the determination of average size of metal nanoparticles/aggregates in silicate glasses based on ultraviolet visible (UV–Vis) spectra fitted with the Mie and Mie–Gans models was reported. Gold ions were diffused into sodalime silicate and borosilicate glasses by field-assisted solid-state ion-exchange technique using the same experimental parameters for both glasses. Transmission electron microscopy was performed to directly investigate the morphology and distribution of the dopant nanoparticles. UV–Vis spectra of the doped glasses showed broad surface plasmon resonance peaks in their fingerprint regions, i.e., at 525 and 500 nm for sodalime silicate and borosilicate glass matrices, respectively. These spectra were fitted with the Mie model for spherical nanoparticles and the Mie–Gans model for spheroidal nanoparticles. Although both the models were developed for colloidal nanoparticles, the size of the nanoparticles/aggregates calculated was accurate to within in both the glass matrices in comparison to the size measured directly from the transmission electron microscope images.