A calibration device for a machine tool is described that includes a base attachable to a machine tool and a calibration artefact, such as a sphere of known radius. A deflection mechanism attaches the calibration artefact to the base and allows movement of the calibration artefact relative to the base when an external force is applied to the calibration artefact. The deflection mechanism also maintains the calibration artefact in a defined rest position relative to the base in the absence of an applied external force. A sensor is provided for sensing the extent of movement of the calibration artefact relative to the base. A method of using the device with a reference tool to accurately determine a position of a calibration artefact is also described.