The effects of carbon content on the tetragonality and magnetic moment of the Fe–C system have been evaluated using the first-principles calculation. Three types of supercell, Fe54C1, Fe54C2, and Fe128C1 (which correspond to Fe-0.40C, Fe-0.79C, and Fe-0.17C mass%, respectively) are used for the calculation. The main results are as follows: (1) The total and mechanical energies of the Fe–C system with carbon atoms at the octahedral sites are smaller than those of the system with carbon atoms at the tetragonal sites. The carbon atom at the octahedral site produces a relatively large expansion in one direction; (2) The tetragonality of the Fe–C system obtained using the first-principles calculation increases linearly with increasing carbon content and agrees well with experimental results. The average magnetic moment of the Fe atom increases with increasing carbon content; (3) The magnetic moment of the Fe atom at the carbon atom nearest neighbor site is lower than that of pure iron and increases with increased distance between the Fe and carbon atoms. The projected density of states exhibits a hybridization with the main contributions being from Fe d and C p states, which leads to the abovementioned decrease in the magnetic moment of the Fe atom. (4) In Fe54C2, the tetragonality and magnetic moment of the Fe atom change with the distance between two carbon atoms, with the tetragonality being 0.981, 1.036, or 1.090. When the Fe–C–Fe pair, which consists of the first carbon atom and its two nearest neighbor Fe atoms, is perpendicular to the second pair, which consists of the second carbon atom and its two nearest neighbor Fe atoms, the tetragonality is 0.981 and does not agree with the experimental value. The mechanical energy is relatively large. On the other hand, when the first pair is parallel to the second pair, the tetragonality is 1.036, which agrees well with experimental data. In this case, the mechanical energy is relatively small. When a straight C–Fe–C pair is formed, the tetragonality is 1.090; (5) In an Fe54C2 supercell, the formation enthalpy is relatively low when the calculated tetragonality is 1.036, and the existence probability under the assumption of a Boltzmann distribution is high. In other cases, the formation enthalpy is relatively high and the existence probability is almost zero; (6) The average magnetic moment of an Fe atom is proportional to volume, but it is not clearly related to the tetragonality. It is believed that the increase in the magnetic moment of the Fe atom by the addition of a carbon atom is primarily due to the magnetovolume effect, and is not due to the tetragonality effect.