In this paper, the virtual air-gap formulae for electromagnetic forces are derived from the generalized forms of equivalent source methods, such as magnetic charge and magnetizing current. The virtual air-gap approach is based on the idea that insertion of infinitesimally thin air gap causes no real variation of electromagnetic field system. For total force calculation of a magnetic body in contact with other ones of ferromagnetic or permanent magnets, the virtual air-gap formulae can employ the conventional force methods, such as Maxwell stress, magnetic charge, and magnetizing current. The virtual air-gap approach is validated by numerical test results.