the size of an interior door frame is determined by the width of the wall into which it is installed. the width of the wall is determined by the width of the stud in the wall and the thickness of the sheet of drywall installed on each side of the wall. a 2 by 4 stud is 3 5 / 8 inches thick. a 2 × 6 stud is 5 5/ 8 inches thick. find the difference in thickness between a firewall contracted with 2 by 6 and drywall that is 3/4 inches thick and a fire Wall constructed with 2 by 4 stud and drywall that is 5/8 inch thick.