A method for incrementally ascertaining a rotation angle of a shaft, in particular of a crankshaft of an internal combustion engine, the shaft being connected to a sensor wheel having teeth and tooth gaps, and at least one sensor being associated with the sensor wheel, the sensor generating a square-wave signal as an output signal which may assume a first value or a second value, and a signal edge being associated with a tooth edge, and the position in time of a counteredge with respect to the edge encoding a direction of rotation, an assignment of a direction of rotation reversal point to a tooth of the sensor wheel or a tooth gap of the sensor wheel being encoded by the position in time of the counteredge with respect to the edge, the rotation angle being ascertained by adding an increment for each signal edge to a counter, after a direction of rotation reversal while a tooth passes the sensor, a half-increment, having a sign of the direction of rotation after the direction of rotation reversal, being incremented once and in the event of a direction of rotation reversal while a tooth gap passes the sensor, a half-increment, having a sign of the direction of rotation prior to the direction of rotation reversal, being incremented once.