We have developed an analysis method to determine the amplitude of inertial motion from surface trajectory data of ARGO profiling floats. This method gives an estimate of inertial current speed and phase from satellite fixes transmitted while a float drifts at the surface, over a time which is typically about one local inertial period. The error of this method is about 3.4 cm/sec for data records with at least 6 positions with accuracy of 500 m, spanning longer than 68% of the inertial period, whereas inertial amplitudes are as large as 40 cm/sec with a median of about 12.0 cm/sec in the Japan/East Sea. Globally distributed ARGO floats could provide useful information on the co-variability of wind-forced inertial currents and mixed layer depths in the ocean.