Carpenters, dealers and designers rely on industry standards to ensure doors are somewhat interchangeable. Differences in width typically occur in 2-inch increments. Differences in height are few.
The majority of interior doors, including bed and bathroom doors, are 80 inches in height, with some exceptions for shorter doors that measure only 78 inches.
The standard width is typically 32 inches for bedroom or bath doors, with exceptions for more narrow doors measuring 24, 28 and 30 inches, and larger doors measuring up to 36 inches, similar to exterior doors.
If the opening is drywall at the top and sides, it's likely been sized for a bifold door. If it's got wood jambs, it's sized for a standard door. The readily available, standard bifold door is usually 80 inches tall. The width of the opening can vary from 24 inches to 72 inches, with individual panels or doors divided equally. For example, a 72-inch bifold door -- or 6-0 -- typically consists of four 18-inch-wide panels or doors.
The rough opening is the stud frame of the opening. It's always larger than the door plus jambs, plus a gap needed for installation purposes. Typical guidelines for sizing a door to a rough opening include adding 2 1/2 inches to the width of the door and 3 1/2 inches to the height. This allows for the gap on both sides and at the top. Shims -- typically cedar -- are tapped into the gap to level and plumb the door inside the opening.
Jamb thickness is one of the most overlooked aspects of door sizing. The most common doorjamb is 4 5/8 inches wide. The formula for measuring is to add the width of the stud -- 3 1/2 inches -- and the thickness of the drywall on both sides -- 1/2 inch -- plus 1/8 inch to ensure that it covers everything. Thus the total of 4 5/8 inches for standard jambs.
Double doors are often used when the rough opening is over 36 inches in width. Height is the same as standard doors -- 80 inches -- but the width is equally divided by two doors, with adequate clearance for the jambs.
Don't Forget the Bevel
Some doors have a 2-degree bevel on the side opposite the hinges -- it's more common on exterior doors. The bevel enables the door to clear the jamb when it's opened or closed. If the door doesn't have a doorknob or hardware already installed on it, check to see if it's beveled for proper installation of the hardware.