A fixture for drilling a pocket hole in a working face of workpieces of different thicknesses is a U-shaped member having a guide leg and a clamp leg parallel to each other and joined together at a bottom end of each by a transverse base leg. An angled guide channel is in the guide leg that has an entrance for a drill bit on an exterior part of the guide leg and an exit on an interior face which opposes the workpiece working face. A plurality of horizontal elongated ribs are spaced vertically apart on the guide leg interior below the guide channel exit and there is a shelf having a face on which a second face of the workpiece that is transverse to the working face is to rest and that has an edge with a plurality of horizontal vertically spaced ribs to inter-engage with the ribs on said guide leg. A screw extends outwardly from the shelf to extend through a slot in the guide leg and a nut fastens the screw against the guide leg to hold opposing ribs of the guide leg and shelf inter-engaged. Selection of the point of inter-engagement of the ribs sets the height of the shelf support for the workpiece relative to the drill guide channel exit.