Open the drawer until it is fully extended.
Place the end of a standard carpenter's tape measure against the back of the drawer on the drawer bottom.
Extend the tape measure along the bottom of the drawer until you reach the front. Bend the tape measure upward so that it is flat against the inside of the drawer front and forms a 90-degree angle. Ensure that the tape measure lies flat along the bottom of the drawer from front to back.
Read the number on the tape measure where it bends at the corner between the drawer front and the bottom.