A method (51) for measuring the mass of an object such as a letter (10) in motion. An apparatus (12) for carrying out the present invention includes a pair of rollers (16, 18) through which a letter passes. One of the rollers (16) is driven by a motor (20) which has an associated encoder (22) to give motor positional information to a controller (24). In a method of the present invention the controller generates a ramping torque signal (44) which is provided to the motor. A friction correction curve (40) is generated and applied to the torque signal, and an average torque value (C) is determined. From the positional information provided by the encoder, acceleration (50) is derived and an inertia value is obtained by dividing the average torque (C) by the acceleration. Prior to obtaining an inertia value with a letter, an initial inertia value is obtained without a letter to get a base inertia. The base inertia is subtracted from the measured inertia with a letter to get a difference .DELTA.l. The mass (M) of the letter is obtained using the formula .DELTA.l=1/2M r.sup.2, where r is the radius of the driven roller.