Device and method to assist in performing chest compressions on a patient or manikin (2) during cardiopulmonary resuscitation (CPR), in which a signal representative of the movement of the chest (3) of the patient or manikin (2) is analyzed in the frequency domain in short-time windows, and the mean depth of chest compressions in each window is estimated from the mean frequency and the amplitudes and phases of the first harmonics of said signal of each window.