Door openings commonly have two king studs and two jack studs. A king stud is one that runs continuously from the top plate to the bottom plate. The jack stud is nailed to the king stud, but it’s shorter as it supports the structural header or the flat 2x material that you use for a door in a non-load-bearing wall.
You may have to install small cripple studs between the top of the header and the top plate. These need to be a continuation of the other king studs on the wall; be sure to maintain the even 16-inch or 24-inch on-center spacing you have for the finished wall material.
The width and height of a rough opening for a door is very important. Your plans may call for a 3-foot-wide door that is 6 feet 8 inches tall. From years of experience, I’m here to tell you that the rough opening should be
at least 38 1/4 inches wide and the height of the opening should be 84 inches.
You must keep in mind that exterior doors have a threshold that’s often at least 1-inch high and that the bottom of this threshold MUST be installed at the same elevation as the top of the finished floor in the house. Rookies often forget this making the rough opening too short.
Height considerations are the same for interior doors. These doors are set on the finished floor and there is almost always a 1-inch air gap under the door after it’s installed.
Just do the math for exterior doors. Here are the things you need to consider: the actual door height, the thickness of the weatherstripping under the door, the thickness of the threshold, the thickness of the finished floor, the thickness of the finished top door jamb, and then add about 1/2 inch wiggle room for shims and other site conditions.