Apparatus for counting and stacking sheets in which document length and multiple fed sheets are detected by examining sheet transmissivity. The light directed through moving sheets fed one at a time through a sensor location is modulated according to sheet transmissivity. The signal for each sheet is compared against the signal for the prior sheet to determine the feeding of multiple sheets. Sheet length is determined through the use of a pair of sensors which provide for counting of sheets which may be fed in a skewed manner. A skew value is used to modify the value obtained when measuring document length to permit accurate counting of both skewed and unskewed sheets. Both hardware and software techniques are utilized to obtain the desired results.