We report the results of the optical spectra modeling for hydrogenated amorphous silicon (a -Si : H ) thin films produced for photovoltaic cell applications, which allowed us to accurately determine the material’s optical gap (E_g) . While for thick films of a -Si : H , as well as for other amorphous semiconductors, E_g is normally estimated from a so called Tauc plot, this is hardly possible for thin films because of the interference effects. We developed a physically founded semianalytical model for the complex dielectric function of a -Si : H , valid below and above the optical gap and containing a small number of adjustable meaningful parameters, including E_g and the characteristic energy scales of the optical transition matrix element distribution and the joint density of states in the absorption tail region. With this model and using the transfer matrix formalism for multilayer optics, we have achieved a good agreement between the calculated and experimental transmittance spectra, which allowed us to self-consistently determine the values of the above parameters. We found that both E_g and the characteristic scale of the subgap absorption tail increase with the hydrogen addition. We have also determined the absorption rate spectra of the films, relevant to any optical spectroscopy of subgap states, such as the photothermal deflection spectroscopy and constant photocurrent measurements.