Frame a door opening right the first time. Enter the door size and configuration to get the rough opening width and height plus the header length to cut.
Rough Opening Width
— in. wide
Width of the framed opening (door width + allowance).Usage Tip
Leave the shim space — framing exactly to the door size leaves no room to plumb and shim the jamb. Confirm the manufacturer rough-opening dimensions before you cut.
THE MATH
Single — RO width = Door width + 2″; RO height = Door height + 2.5″
Double — RO width = (2 × Door width) + 2.5″; RO height = Door height + 2.5″
Header length = RO width + 3″
Double — RO width = (2 × Door width) + 2.5″; RO height = Door height + 2.5″
Header length = RO width + 3″
The width allowance covers the jamb (about ¾″ per side) plus shim space; the height allowance covers the head jamb, head gap, and finished-floor clearance.
Header length adds 1.5″ for each jack stud the header bears on.
Header length adds 1.5″ for each jack stud the header bears on.
These are common pre-hung allowances, not a universal standard — door and pre-hung unit makers publish exact rough-opening sizes, so confirm against the spec sheet for your specific unit before framing.
For a slab fitted into an existing jamb, frame to the jamb maker rough opening instead.
Keep the opening plumb, level, and square, and do not frame tight to the unit — the shim gap is what lets you adjust it.
For a slab fitted into an existing jamb, frame to the jamb maker rough opening instead.
Keep the opening plumb, level, and square, and do not frame tight to the unit — the shim gap is what lets you adjust it.