An apparatus for determining the stiffness of mail pieces includes a bar or similar deflector for contacting a mail piece conveyed past the bar by a conveyor system, and vibrates when in contact with a passing mail piece, either due to the impact of the mail piece or by the action of a device which causes the deflector to vibrate before the mail piece hits it. A sensor measures a parameter indicative of change in the vibrations of the deflector and generates a signal indicative of the measured parameter. A processor, e.g. programmable controller or circuit, receives the signal from the sensor and determines a stiffness value for the mail piece based on decay in vibrations of the deflector caused by contact with the mail piece.