A screw counter includes a first interface, a second interface, and a processing unit. The first interface connected to a screwdriver, the screwdriver is adapted for screwing a predetermined total number of screws into a workpiece to perform a screwing operation on the workpiece. The second interface is connected to a detecting unit. The detecting unit detects a start time and a finish time of the screwing operation. The processing unit counts a variable, each time when the processing unit is changed from a first state to a second state, the variable adds one; a total added value of the variable is calculated during a time interval between the start time and the finish time to acquire a total number of screws which are completely screwed into the workpiece.