Rough opening sizes for doors and windows

Rough opening sizes for prehung interior doors

A prehung door comes with its slab already hung in a frame. The rough opening is the framed hole it slides into, and it needs to be a little bigger than the frame so you can shim the jambs plumb and square. Framers size it from the door: width plus 2 in, and 82-1/2 in high for a standard 6'8" (80 in) door.

We checked the convention against JELD-WEN's interior prehung instructions (JII109). They do not give a door-plus figure; they ask for a rough opening 1/2 in wider and 1/2 in taller than the outside of the door frame. The 2 in rule is meant to give that, but frames vary, so measure the unit you are buying, or its listed rough opening, before you cut a header. Doors 8'0" tall take 98-1/2 in by the same rule.

Exterior door and window rough openings

Exterior doors and windows are sold by unit size, and the rough opening comes from the manufacturer, not from a rule of thumb. Two makers' instructions show the range:

So a vinyl window whose frame measures 36 × 48 in needs at least 36-1/2 in × 48-1/2 in; the same size in aluminum-clad wood needs 36-3/4 in × 48-3/4 in. Other makers publish their own clearances, and the window's own instructions govern. Get them, or the order confirmation, before you frame.

For exterior doors, JELD-WEN's instructions warn that a sill pan and other flashing take height out of the opening, so allow for them. Pella asks for 1/2 in to 1 in of extra height over the frame for the same reason.

How the header, jacks and kings frame the opening

Every opening gets 2 king studs, which run full height from the bottom plate to the top plates, and 2 jack (trimmer) studs nailed inside them. The jacks carry the header, and the space between the jacks is the rough opening width. The header is cut to the rough opening plus 3 in, so it bears 1-1/2 in on each jack: 41 in for a 36 in door.

For a door, the bottom plate runs through while the wall is built and is cut out of the opening once the wall stands. With an 82-1/2 in opening measured from the subfloor, each jack is 82-1/2 in − 1-1/2 in of plate = 81 in long.

Above the header, cripples fill the space up to the top plates at the stud spacing. On an 8 ft wall built with 92-5/8 in precut studs, the underside of the double top plate is 94-1/8 in up, which leaves 11-5/8 in over an 82-1/2 in opening. A header of 2×12s (11-1/4 in deep) nearly fills that, so some framers use a solid header of that depth on 8 ft walls and need no cripples over it; the calculator still counts them, and they make useful blocking. On a 9 ft wall the space is 23-5/8 in, room for a header and cripples, and still 7-5/8 in over an 8'0" door. A window adds a rough sill at the bottom of the opening with cripples under it, so the 36 in vinyl window above takes 6 cripples at 16 in on center, half above the header and half below the sill.

What to check before you frame the opening

• The unit, not the label. Measure the frame of the door or window you have, or get the maker's listed rough opening. Nominal sizes like 3'0" × 6'8" describe the slab, not the frame.
• Wall thickness. A 2×4 wall with 1/2 in drywall on both sides is 4-1/2 in thick. Interior door jambs are sold in widths to suit; order the jamb to the finished wall, and a wider one for 2×6 walls or thicker board.
• Floor finish. Hardwood, tile or a thick underlay raises the floor under the door. Decide the floor before you set the header height.
• Swing and clearance. Leave room beside the opening for the casing and, on the latch side, for a light switch.
• Square, plumb and level. Check the diagonals and the jacks before setting the unit. JELD-WEN's instructions list the tolerances, and an opening out of square makes the door bind no matter how well it is shimmed.

For stud counts by wall length, see the studs per foot chart.