Graphene quantum dots (GQDs) and their colloidal suspensions have been attracting attention for various applications such as biomedical imaging or energy harvesting. Optical properties need to be calculated for quantum dots’ effective use in different applications. Spectral absorbance for dilute colloidal suspensions of graphene quantum dots in water is predicted theo- retically by several methods including Rayleigh theory, Rayleigh Debye Gans approximation for fractal aggregates (RDG-FA) and discrete dipole approx- imation (DDA) within visible range of light (450- 700 nm). Confinement effect is observed while shrinking from 3 dimensional bulk material to quan- tum dots, and when the size of GQD is smaller than 10 nm, confinement effect becomes remarkable.The sizes of the inspected colloidal suspensions of quantum dots in water are reported to be 5 nm in average. After calculating optical properties of graphene quantum dots by using these approximations, the predicted spectral absorbances are compared to the experimental results. Absorption spectra of colloidal suspensions of GQDs in water with concen- trations ranging from 10000 ppm to 80000 ppm are obtained by Avantes Spectrometer using halogen lamp illumination and AvaSoft software. The size distribution and therefore the aggregation level of the quantum dots are identified through comparisons of the predicted and the measured spectral absorbances.