Abstract: A surgical robot calibration device configured to be used when calibrating a surgical robotic system to perform a minimally invasive procedure through a natural orifice, the surgical robotic system comprising a surgical robotic arm and a surgical instrument having a rigid linear shaft, the surgical robot calibration device comprising a resistive spacer configurable to hold a calibration port in a fixed position spaced from the natural orifice, such that when the calibration port is held in the resistive spacer, the surgical instrument is insertable into the natural orifice via the calibration port to enable a fulcrum about which the surgical instrument pivots whilst the surgical instrument is inserted into the calibration port to be determined.