The split ratio of an interior rotor permanent-magnet machine, which relates the rotor diameter to the total motor diameter, can be optimized with the goal of generating maximum torque for a given motor volume. For that purpose, thermal constraints have to be considered that limit the producible magnetomotive force and, consequently, the torque output. Both a maximum local current density in the coils and a maximum global copper loss density related to the motor surface are respected. In this paper, the analytical calculations for torque and split ratio are derived and the influence of these thermal constraints is elaborated.