The force capability of any kinaesthetic haptic device strongly depends on its kinematic structure, actuator gear ratios, and actuator torque ranges. Traditionally, performance metrics of haptic devices are reported in terms of peak force, workspace volume, and condition number. But since the force distribution across the workspace strongly depends on the position of the end-effector, and the gear ratio of the actuation unit, traditional metrics cannot are not useful when comparing performance of different haptic devices. This paper introduces the concept of local force isotropy as a new performance metric corresponding to the greatest force that can be generated at the end-effector in any direction at any given point in the workspace. The maximum isotropic force is independent of force direction and kinematic structure, therefore providing a more realistic description of device’s force capability. In addition, the proposed metric describes the maximum isotropic force thought the workspace by considering the actuator torque and gear ratios. The latter is not captured by the condition number method. The proposed method can be used to compare the force output of haptic devices having different kinematic structures, providing a more realistic performance metric than the widely used concept of peak force. The paper uses this method to evaluate the force output capabilities and distortions of a 3-DOF commercial haptic device throughout its workspace. The results suggest that the proposed method is a valuable tool in design optimization and comparison of haptic devices.