A surgical system comprising: a surgical instrument having a variable speed motor and a tool tip; a controller for providing power to the surgical instrument, the controller operable to monitor a motor operation of the instrument, the controller comprising a processor and a memory, the memory operable to store information, including information representing the motor operation of the instrument; an instrument tracker coupled to the instrument; a patient tracker to be coupled to a bone; and a navigation system comprising a localizer; the navigation system operable to store information representing the surgical instrument, and information representing the bone, in a virtual space; the localizer operable to register a location of the instrument tracker and a location of the patient tracker relative to a localizer coordinate system, LCLZ, and to gather information about the location of the instrument and the bone in cooperation with the instrument tracker and the patient tracker, respectively; and the navigation system operable to track the location of the instrument and the location of the bone in the virtual space based on information gathered by the localizer; wherein the controller and the navigation system are in electronic communication and characterised in that the controller and the navigation system are configured to cooperate to: determine, based on a change in the motor operation, a time of contact between the tool tip and the bone; and determine, at the time of contact, a tool-to-bone offset as a distance between the tracked location of the tool tip and the tracked location of the bone. a surgical instrument having a variable speed motor and a tool tip; a controller for providing power to the surgical instrument, the controller operable to monitor a motor operation of the instrument, the controller comprising a processor and a memory, the memory operable to store information, including information representing the motor operation of the instrument; an instrument tracker coupled to the instrument; a patient tracker to be coupled to a bone; and a navigation system comprising a localizer; the navigation system operable to store information representing the surgical instrument, and information representing the bone, in a virtual space; the localizer operable to register a location of the instrument tracker and a location of the patient tracker relative to a localizer coordinate system, LCLZ, and to gather information about the location of the instrument and the bone in cooperation with the instrument tracker and the patient tracker, respectively; and the navigation system operable to track the location of the instrument and the location of the bone in the virtual space based on information gathered by the localizer; the navigation system operable to store information representing the surgical instrument, and information representing the bone, in a virtual space; the localizer operable to register a location of the instrument tracker and a location of the patient tracker relative to a localizer coordinate system, LCLZ, and to gather information about the location of the instrument and the bone in cooperation with the instrument tracker and the patient tracker, respectively; and the navigation system operable to track the location of the instrument and the location of the bone in the virtual space based on information gathered by the localizer; wherein the controller and the navigation system are in electronic communication and characterised in that the controller and the navigation system are configured to cooperate to: determine, based on a change in the motor operation, a time of contact between the tool tip and the bone; and determine, at the time of contact, a tool-to-bone offset as a distance between the tracked location of the tool tip and the tracked location of the bone. determine, based on a change in the motor operation, a time of contact between the tool tip and the bone; and determine, at the time of contact, a tool-to-bone offset as a distance between the tracked location of the tool tip and the tracked location of the bone.