Framing studs are used to create both interior and exterior walls, with drywall or siding then attached to the wall frame. The frame includes horizontal studs at the top and bottom (called plates). Parallel, vertical studs are installed between the plates and centred, 16 inches apart.
Standard Stud Size
A standard wood framing stud, known as a 2-by-4, is generally 8 feet long. Some studs are pre-cut to a 92 5/8-inch length, so a plate can be attached to the top and bottom of the vertical studs to create a wall frame that's 8 feet (96 inches) tall--the standard height of a room.
Dressed Size Vs. Rough Size
Though the standard wood stud is called a 2-by-4 (i.e., 2 inches thick and 4 inches wide), in actuality the stud is 1 1/2 inches thick and 3 1/2 inches wide. This loss of 1/2 inch from both the thickness and width measurements is due to the planing and sanding of the wood stud at the saw mill. The stud's final thickness and width is known as its dressed size. However, the pre-planing and sanding size is always listed as its measurement (known as its rough size).
Larger Width Studs
Wood studs made from 2-by-6-inch lumber (2 inches thick, 6 inches wide) are also used, when a stud width larger than 3 1/2 inches is needed in the wall cavity for the purpose of running electrical cables, plumbing pipes or thicker insulation. These larger width studs are also smaller in reality than their rough size suggests, with a 1 1/2-inch thick and 5 1/2-inch wide dressed size.
Take Care When Measuring/Fitting Wall Studs
Care must always be taken when installing 2-by-4 or 2-by-6-inch wall studs, due to the difference between the dressed and rough sizes of the lumber. Measure the actual width and thickness of the studs, instead of merely going off their given rough size dimensions. Also purchase the correct length of stud to fit the room if the ceilings are to be exactly 8 feet high (note that longer studs are also available for higher ceilings).