Add 2 ½” to door width.
Rough opening for 28 inch pre hung door. The frame around the prehung door is typically a thickness of ¾” on both sides, resulting in a total additional width of 1 ½”. Add 2 ½” to door height. Rough opening for 32″, 28″, 30″, 24″, 34″ & 36 inch door.
Standard door heights are 6/6 (78”) and 6/8 (80”). Add 2 ½” to door height. 24, 28, 30, 32, 34 & 36 in inches for width.
Double door unit with astragal. Now all you need to know is how to properly frame your door rough opening. Door casing will cover the gap between the rough opening the door frame.
In addition, special sizes can sometimes be custom ordered. The standard height of the rough opening for a door is the door height plus 2. Add 2” to door width.
Multiply the width by 2 then and 2”. In theory, that means your opening should be one. There are six standard door sizes: