There are six standard door sizes:

Rough opening for 28 inch pre hung door. Be sure to allow for about a 1½ inch larger opening than the standard door sizes. Now all you need to know is how to properly frame your door rough opening. Add 2 ½” to door width.

Add 2 ½” to door height. The standard height of the rough opening for a door is the door height plus 2. Double door unit with astragal.

Bob schmidt shows you how to accurately frame for an interior door showing you how to measure a door and allow for proper clearance while framing also how to. In addition, special sizes can sometimes be custom ordered. Standard door heights are 6/6 (78”) and 6/8 (80”).

Also know how wide is a 2 8 door. Multiply the width by 2 then and 2”. The frame around the prehung door is typically a thickness of ¾” on both sides, resulting in a total additional width of 1 ½”.

24, 28, 30, 32, 34 & 36 in inches for width. Add 2” to door width. In theory, that means your opening should be one.