Toner hopper (1) of a printer (70) has a stirring paddle (3) with an encoder wheel (30, 50) mounted on paddle shaft (5). The drive connection to paddle (3) is through a torsion spring (60). Data processing apparatus determines paddle acceleration or deceleration (90, 94, 98, 102) and executes a table look-up to determine scaled amounts of subsequent movement (92, 96, 100, 104). Steady state movement is a unitary (unscaled) amount (106). These amounts are totaled (93) and used to define yield at the torsion spring, which corresponds to amount of toner in the hopper. This eliminates hardware in previous embodiments at the drive motor to signal actual rotation of the drive motor.