The solubility product of NbC in low alloyed steel is computed from electronic density functional methods including the effects of electronic, vibrational, and magnetic excitations. Although many simplifications are made in the computations, agreement with experimental data is within the scatter of the latter. The T = 0 K terms dominate in the determination of the solubility product but vibrational and magnetic contributions play a significant role also while electronic excitations can be neglected. Supercell calculations were shown to be poorly suited for determination of embedding enthalpies of solutes in bcc Fe.