Also know how wide is a 2 8 door.
Rough opening for 28 inch pre hung door. Add 2” 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.
Standard door heights are 6/6 (78”) and 6/8 (80”). Add 2 ½” to door height. Now all you need to know is how to properly frame your door rough opening.
In theory, that means your opening should be one. In addition, special sizes can sometimes be custom ordered. The frame around the prehung door is typically a thickness of ¾” on both sides, resulting in a total additional width of 1 ½”.
Door casing will cover the gap between the rough opening the door frame. Double door unit with astragal. There are six standard door sizes:
Multiply the width by 2 then and 2”. Add 2 ½” to door width. Be sure to allow for about a 1½ inch larger opening than the standard door sizes.