Abstract: The present invention relates to an apparatus for pneumatic length measurement comprising a pre-nozzle and a measurement nozzle through which the medium used for the measurement is directed onto an impact plate, a first pressure sensor arranged in front of the pre-nozzle, a second pressure sensor arranged between pre-nozzle and measurement nozzle, means for the determination of the distance of the impact plate from the measurement nozzle from the change of the pressure between pre-nozzle and measurement nozzle taking account of the pressure measured by the first pressure sensor and means for the calibration of the apparatus in a given measurement range, wherein, in order to ease the calibration, the calibration means operate with a calibration function which is constructed on the assumption of constancy of the outflow number of the pre-nozzle and of the outflow number of the measurement nozzle.