Abstract: A positioner for a carton sizer is provided. The positioner includes a housing and a latch. The housing houses the latch. The latch is pivotally connected to the housing. The latch may be pivoted between a latched position and an unlatched position. The positioner may be removably attached to a mast of a carton sizer at a selected height along the mast. The latch may engage the mast in the latch position to hold the positioner to the mast. The latch may disengage from the mast in the unlatched position. The carton sizer includes a handle and a cutter. The handle is connected to the mast. The positioner may engage a carton to provide support for the cutter against the carton during the cutting of the carton.