We present a new automatic code (ARES) for the determination of equivalent widths of absorption lines present in stellar spectra. We also describe its use for the determination of fundamental spectroscopic stellar parameters. The code is written in C++ based on the standard method of determining EWs, and is available for the community. The code automates the manual procedure that the users normally carry out when using interactive routines such as the splot routine implemented in IRAF. We test the code using both simulated and real spectra with different levels of resolution and noise comparing its measurements to the manual ones obtained in the standard way. The results shows a small systematic difference, always bellow 1.5mA. This can be explained by errors in the manual measurements due to subjective continuum determination. The code works better and faster than others tested before. This code will be used to analyze spectra from the HARPS sample to search extrasolar planets.