Width is important if you're replacing a door jamb. The correct width allows the jamb to transition smoothly to both sides of the wall. Correct door jamb width includes the thickness of the drywall, with a bit of play to compensate for uneven framing or wall coverings.
Door jambs are the vertical sides of the door frame. They appear built in, but are actually screwed to the rough framing of the house, with smooth, finished casing on both sides and at the top. The casing hides the gap between the jamb, the rough framing and wall.
The doorstop is the raised portion of the jamb, or the strip of wood in the center of the jamb. It's function is to give the door something to close against, and is typically added after you install the jamb.
Pre-hung Versus Piece by Piece
The majority of door installations employ pre-hung jambs. This means that the door and jamb unit are pre-assembled at the factory, and installed as a completed unit. A jamb installed piece by piece is more difficult to install, but the process of measuring it is exactly the same as a pre-hung unit.
Measure With Math
If you're measuring from scratch it's called a rough opening measurement, with exposed studs on both sides and at the top. Even without measuring you can do the math: Most framing studs are 3 1/2 inches in wide, with 1/2-inch-thick drywall on both sides, for a total thickness of 4 1/2 inches. It's a common practice to add another 1/8 inch for some leeway, to ensure the jamb covers adequately, making the average door jamb 4 5/8 inches wide.
Measure a Rough Opening
If you're not sure if you have a standard width opening, measure the distance from wall to wall on the outside, and add 1/8 inch. Measuring it this way ensures that, in the event that the drywall is thicker or thinner, or paneling or other wall coverings have been applied, the jamb is the right size.
Measure Jamb Thickness
Door jamb thickness is almost always 3/4 inch. Measure this by removing the casing from one side to reveal the side of the door jamb. Casing is easy to pry loose with a pry bar, or even a screwdriver. There will be a gap between the jamb and the rough framing. The gap should contain small cedar wedges that are used to level and plumb the jamb. Measure the jamb only.
Measure For Trim
Trim around a jamb is typically referred to as door casing. Standard door casing is typically 2 1/4 inch wide, and about 5/8 inch thick. When measuring for casing length, add at least 2 1/2 inches -- 3 inches is best -- to the height of the opening for the two vertical pieces, and 6 inches for the horizontal piece across the top. Home supply stores may have standard lengths pre-cut for door jambs, or about 84 inches. The typical door opening is usually about 81 inches. The added 2/1/2 to 3 inches of length allows for cutting and mitering the casing.