A method of calibrating a measurement probe (10) mounted on a machine is described. The measurement probe (10) has a stylus (14) with a workpiece contacting tip (16). The method comprises determining a probe calibration matrix that relates the probe outputs (a,b,c) to the machine coordinate system (x,y,z.). The method comprising the steps of scanning a calibration artefact (18) using a first probe deflection (d1) to obtain first machine data and using a second probe deflection (d2) to obtain second machine data. The first and second machine data are used to obtain a pure probe calibration matrix in which any machine errors are substantially omitted. Advantageously, the method determines the pure probe matrix numerically based on the assumption that the difference between the first and second machine position data is known.