Photons propagating within photonic crystals can be regarded as quasi-particles having an effective mass. Methods of calculation of this mass from the photonic density of mode are presented for the case of one-dimensional photonic crystal, both from infinite and finite structure. The model is implemented to calculate the influence of the number of unit cells and of absorption, for bilayered Bragg structure used as reflectors in the soft-x-ray domain. The physical meaning of the effective photon mass concept is discussed.