Composition, chemical disorder, and magnetism significantly affect the volume and bulk modulus of iron–silicon (Fe–Si) alloys at ambient pressure. Here, we computed the equations of state for bcc-like (ordered B2 and disordered bcc) Fe–Si alloys available up to the inner-core pressure using the first-principles Korringa–Kohn–Rostoker method. Ferromagnetic (FM) and nonmagnetic (NM) states over a wide composition range, from Fe to FeSi, were investigated. The results revealed that magnetism and chemical disorder increased the volume and decreased the bulk modulus even at high pressures. Comparing the results with the preliminary reference Earth model, we found that an unrealistically large temperature gradient is required if the inner core is composed of a bcc-like Fe–Si alloy.