The typical interior wall thickness in newer construction using 2-by-4 studs and half-inch drywall is 4.5 inches. 2-by-6 studs are typically used in walls that contain plumbing, making those walls 6.5 inches. Exterior wall thickness varies, depending on the exterior finish, siding and brick facing.
The nominal dimension of 2-by-4 (inches) refers to the size of green, rough-sawn studs. Drying and planing reduce the finished size to the current standard of 1.5 by 3.5 inches. Interior walls in older construction may differ. Pre-1970 studs were thicker, at 3 5/8 instead of 3.5 inches, and the use of lath and plaster is somewhat thicker than half-inch drywall. Earlier still, green, rough-sawn 2-by-4s were delivered and planed to a finished size on site to a standard determined only by the individual carpenter. The best way to figure the wall thickness in an existing house is to measure the width of the door frame and subtract the thickness of the trim.