The article deals with designing rotor disks of a double-sided axial flux permanent magnet machine, based on their deflection, due to the pull forces between opposite rotor disks, caused by the permanent magnets. Permanent magnets cause the magnetic flux density to flow between the permanent magnets on opposite rotor disks and its size determines the size of the pull force. That is why it is essential to determine the size of the axial component of magnetic flux density in the middle of the clearance between opposite rotor disks (fictitious air gap) in order to calculate the deflection of the rotor disks. The article presents a new method for determining the size of the axial component of magnetic flux density, based on a simplified finite element method and an approximation method as well as modified equations for determining the deflection of rotor disks. The method is used to determine the pull forces between rotor disks and based on the deflection of the disks an optimum rotor disk thickness can be selected in order to reduce the weight of the machine.