The magnetic field occurring in the air gap of a surface permanent magnet (PM) machine is analytically modelled. The methodology is based on the use of some “field functions”, each of one representing the field waveform due to a particular source (PM or current) or anisotropy effect (slotting). Such field functions are identified by selective FEM magneto-static 2D-simulations, with a limited time consumption. The analytical model allows to reconstruct the air gap field with great accuracy, as verified by the comparison with numerous FEM simulations, both in no load and in loaded operating conditions.