How Do I Determine the "Jamb Width" for a Door?

Determining the jamb width for a door can be tricky. For most interior applications it is  simply a matter of measuring the overall wall thickness. For instance, a standard wall constructed with 2 x 4 studs and 5/8" sheetrock on both sides would have a wall thickness of 4-13/16", that is 3-9/16" (the net width of the 2 x 4 stud) plus 1 1/4" (5/8" sheetrock times 2 sides). To determine the wall thickness of an existing interior door you will need to measure the overall jamb width, excluding the casings.

For exterior doors, determining the jamb or frame width is dependent upon what type of siding and exterior trim is being used. In situations where the exterior trim is being applied over the exterior siding, then it is simply a matter of measuring the entire wall thickness. In other situations where the siding material (such as stucco) is butting up to the exterior trim, then the thickness of the exterior material would not be included in the jamb width.