A method of calibrating a probe is disclosed said probe being mounted on a machine and having a stylus with a workpiece contacting tip, comprising calculating calibration information for the probe for a first orientation of the probe, and rotating the calibration information by an angle to obtain a probe calibration information for when the probe is oriented by that angle with respect to the first orientation. Also disclosed is a method of calibrating a probe during a measurement process. The calibration information may include a vector which relates probe head axes to machine axes; a calibration matrix; datum data; an inertial matrix. The stylus tip may be datumed at the orientation of the probe or inferred from datum information obtained at different orientations. The rotation step may be carried out by a software/computer program which may be stored on a controller for the machine.