Interior door rough opening — door size plus 2 inches wide and 2½ inches tall for a pre-hung
The hole in the wall is bigger than the door. A standard pre-hung / slab needs a rough opening = door width + 2″ wide × door height + 2½″ tall — the extra inch each side and 2½″ at the head are the jamb plus shim gap that lets the frame be set plumb and level. Order the door by nominal size; frame to the RO.
What this diagram shows
A framed wall opening with a pre-hung door nested inside it, showing why the rough opening is bigger than the door. A 30-inch-wide by 80-inch-tall door needs a rough opening framed 32 inches wide and 82½ inches tall: the extra 1 inch on each side and 2½ inches at the head leave room for the jamb plus a shim gap so the finished frame can be set plumb and level. The rough opening is measured between the trimmer (jack) studs and the underside of the header, not to the door. So the rule for a standard pre-hung or slab is rough opening width equals door width plus 2 inches, and rough opening height equals door height plus 2½ inches. Order the door by its nominal size; frame the wall to the larger rough opening.
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