Building with standard sized lumber helps you to have predictable dimensions in your structure and makes later stages of wallboard and finish trim faster and more efficient. Construction lumber is manufactured in a number of standardised sizes to make construction easier for builders. The size of a wall stud depends on how thick the builders want the wall to be.
Stud sizes are referred to in full-inch terminology as a matter of convenience, even though they are actually smaller. The two most common dimensions for wall studs are 2-by-4 and 2-by-6. However, because the studs are planed down at the factory, a 2-by-4 is actually 1 1/2 by 3 1/2 inches, and a 2-by-6 is actually 1 1/2 by 5 1/2 inches. Studs that are actually 2-by-4 inches and 2-by-6 inches can be bought at sawmills before they have been planed, but their dimensions are more subject to variation.
When stud building first became common in the early 20th century, 2-by-4 studs were the standard. Since that time, because of concerns about energy use, 2-by-6 construction is becoming more common. Building walls with studs that are 5 1/2 inches wide instead of 3 1/2 inches wide allows space for two more inches of insulation in the wall, something that greatly improves the thermal efficiency of the building.
The length of wall studs varies depending on the desired ceiling height of the room. Most residential construction features rooms with ceilings that are between eight and 10 feet in height, therefore these are the most common stud lengths. Note that an 8-foot stud must be trimmed by three inches to accomodate a top and bottom plate for a typical wall. Pre-cut studs are often available in these lengths. At most lumber yards, 2-by4s and 2-by-6s can be purchased in lengths anywhere up to 20 feet.
Wood and Steel
Steel studs are becoming as common as wood studs in the construction industry. To conform to the standards that have been created by the use of wood studs, steel studs are manufactured to the same dimensions: 1 1/2 inches in thickness and either 3 1/2 or 5 1/2 inches in width.