From the first principles, the influence of carbon content on the magnetic moment of the BCC and BCC Fe-C systems was evaluated. The systems Fe-0.40 C, Fe-0.79 C and Fe-1.19 with mass% were considered. It is shown that the tetragonality of the Fe-C system increases linearly with increasing carbon content and is in good agreement with experimental results. The average magnetic moment on Fe atoms also increases linearly with increasing carbon content. At the same time, the magnetic moment of Fe atoms located in the immediate vicinity of carbon atoms is less than in pure iron, and the atoms of the second coordination sphere, on the contrary, have more. It is shown that this is primarily due to the magneto-volume effect, whereas the effect of tetragonal distortion of the lattice is secondary