An automatic toilet flushing system, for use with conventional toilets, includes a sensor, an actuator and a flushing mechanism. The actuator has a gear train, a motor, a pivotal arm connected to the gear train, and a power source connected to the motor for rotating the shaft of the motor which rotates the gear train for pivotal movement of the arm. The flushing mechanism includes a flapper valve and the pivotal arm is connected to the flapper valve via a bead chain and a handle swivel or a tab mounted on the pivotal arm. The sensor may replace the flush handle or the flush handle and the sensor may both be provided. A kit and a method for converting a manual flushing system to the automatic toilet flushing system are also provided.