When 2-by-4 studs and half-inch drywall are used, the typical wall thickness inside a newer home is 4.5 inches. Most plumbing walls are made of 2-by-6 studs, which make the walls 6.5 inches thick. The thickness of the outside walls depends on the outside finish, siding, and brick facing.
The size of rough-sawn, green studs is what the nominal size of 2×4 inches refers to. After drying and planing, the finished size is now 1.5 by 3.5 inches, which is the standard. Older buildings may have different kinds of interior walls. Before 1970, the studs were 3 5/8 inches instead of 3.5 inches, and lath and plaster was used instead of half-inch drywall. Even before that, rough-sawn, green 2-by-4s were brought to the job site and planed to the final size, which was set by the individual carpenter. In an existing house, the best way to figure out how thick the walls are is to measure the width of the door frame and take away the thickness of the trim.