The precision chambers of the ATLAS Muon Spectrometer are built with Monitored Drift Tubes (MDT). The requirement of high accuracy and low systematic error, to achieve a transverse momentum resolution of 10% at 1 TeV, can only be accomplished if the calibrations are known with an accuracy of 20 μm. The relation between the drift path and the measured time (the socalled r-t relation) depends on many parameters (temperature T, hit rate, gas composition, thresholds,...) subject to time variations. The r-t relation has to be measured from the data without the use of an external detector, using the autocalibration technique. This method relies on an iterative procedure applied to the same data sample, starting from a preliminary set of constants. The required precision can be achieved using a large (few thousand) number of non-parallel tracks crossing a region, called calibration region, i.e. the region of the MDT chamber sharing the same r-t relation.