Interior Door Rough Opening: Sizing and Framing It Right

Get the interior door rough opening wrong by even half an inch and you will fight the door for the rest of its life with shims, planing, and a latch that never quite catches. The rough opening is the raw framed hole in your wall, measured stud to stud and header to subfloor, sized deliberately larger than the door so you have room to plumb and square a prehung unit inside it. The universal rule is simple: rough opening equals door slab width plus 2 inches, and door height plus roughly 2.5 inches.

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The Standard Formula

For a prehung interior door, framers use a reliable pair of formulas that account for the jamb thickness and shimming space:

• Rough opening width = door slab width + 2 inches
• Rough opening height = door slab height + 2.5 inches (measured from finished floor, or add for subfloor)

The extra 2 inches of width covers two 3/4-inch jamb legs plus about a half inch total of shim space. The height allowance leaves room for the head jamb, shims, and a finished floor thickness that may arrive after framing.

Rough Opening Sizes by Door Width

Standard interior doors are 80 inches tall (6 feet 8 inches). Here are the rough openings for common widths:

• 24-inch door (2/0): rough opening 26 inches wide by 82.5 inches high
• 28-inch door (2/4): rough opening 30 inches wide by 82.5 inches high
• 30-inch door (2/6): rough opening 32 inches wide by 82.5 inches high
• 32-inch door (2/8): rough opening 34 inches wide by 82.5 inches high
• 36-inch door (3/0): rough opening 38 inches wide by 82.5 inches high

Note that a “30-inch door” refers to the slab, so a 30-inch bedroom door needs a 32-inch rough opening. Mixing up slab size and rough opening is the single most common framing mistake I see on DIY jobs.

Building the Opening Correctly

An interior door rough opening is framed with a specific set of members. From the outside in you have king studs, trimmer (jack) studs, a header, and a sill or nothing at the floor for a door:

1. Set your king studs full height, nailed to the top and bottom plates.
2. Cut trimmer studs to the header height and nail them to the inside face of each king stud.
3. Set the header on top of the trimmers. For a non-load-bearing interior wall, a flat 2×4 or doubled 2×4 header is common.
4. Add cripple studs above the header up to the top plate.

Keep the trimmers plumb and the header level. If the opening is racked out of square before the door goes in, no amount of shimming saves it.

Header Height and Floor Considerations

Frame the top of the rough opening at 82.5 inches above the finished floor for a standard 80-inch door. If you are framing on a subfloor before flooring goes down, account for the finished floor thickness. A 3/4-inch hardwood or a tiled floor with backer eats into your clearance, so measure from where the finished surface will land, not the bare plywood.

Undercut matters too. Prehung doors are usually set to clear the finished floor by about 1/2 to 3/4 inch for carpet or air return, so the height allowance gives you room to raise the whole unit on shims.

Measuring an Existing Opening

Replacing a door in an existing wall? Pull the old casing and jamb to expose the framing, then measure the actual rough opening at several points:

• Width at the top, middle, and bottom, using the smallest number
• Height on both the left and right sides
• Diagonal corner to corner both ways; equal diagonals mean the opening is square

If your measured rough opening is 34 inches wide, buy a 32-inch prehung door. If it is only 32 inches, you either need a 30-inch door or you must reframe to gain the extra 2 inches.

Double Doors and Wider Openings

Double doors, French doors, and wider passages follow the same logic but with doubled math. For a pair of doors, add both slab widths together, then add the 2 inches for jambs and shim space. A set of two 30-inch French doors, for example, needs a rough opening of roughly 62 inches wide (60 inches of door plus 2 inches). Because the header now spans a wider gap, load-bearing walls demand a correspondingly larger header, so a wide opening in a bearing wall almost always requires an engineered or doubled-dimensional-lumber header sized to the span.

Wide openings also flex more, so keep the trimmers dead plumb and the header perfectly level; a small error over 60 inches shows up as a noticeable gap where the two doors meet in the middle. Prehung double-door units come with detailed rough-opening specs from the manufacturer, and you should follow those exact numbers rather than the general formula when they differ.

Accounting for Wall Thickness and Jamb Depth

Width and height get all the attention, but jamb depth matters too. Standard interior jambs are milled for a 2×4 wall with half-inch drywall on each side, giving a jamb depth of about 4-9/16 inches. If your wall is thicker, a 2×6 plumbing wall, or has thicker wall board, a standard prehung jamb will not reach flush to both faces and the casing will not sit right.

For non-standard walls, order a prehung door with the correct jamb depth or plan to add jamb extensions, thin strips that build the jamb out to meet the wall surface. Measure your actual wall thickness, from finished face to finished face, before ordering a prehung unit. This detail catches many first-time remodelers who nail the width and height but overlook that their old plaster walls are thicker than modern drywall construction.

Rough Opening for Special Doors

Pocket doors and bifolds break the standard formula. A pocket door needs a rough opening roughly double the door width plus a couple inches to swallow the door into the wall cavity, so a 30-inch pocket door wants about a 62-inch rough opening. Bifold doors follow the standard door-plus-2-inch rule for the finished opening but read the specific hardware kit instructions, since track hardware varies.

Understanding the Jamb and Shim Space

To grasp why the rough opening is sized the way it is, picture what fills that gap. A prehung door arrives as a slab already hung on a three-sided jamb. The jamb legs are typically 3/4-inch nominal material, so two legs consume about 1.5 inches of your width. That leaves roughly a half inch of total shim space, a quarter inch on each side, which is exactly what a carpenter needs to plumb the door in an imperfect opening.

Shims are thin tapered wedges you slide behind the jamb at the hinge and latch points to hold the door plumb and square regardless of how true the framing is. Without that half inch of play, you cannot straighten a door in a slightly racked opening, and the door will bind or the latch will miss the strike. Too much play, on the other hand, means excessive shimming and gaps the casing struggles to hide. The two-inch rule threads that needle precisely.

Load-Bearing vs. Non-Load-Bearing Walls

The header sizing depends entirely on whether the wall carries weight from above. Most interior partition walls running parallel to the floor and ceiling joists are non-load-bearing, and a simple flat 2×4 or a doubled 2×4 on edge suffices as a header for a standard door opening.

If the wall is load-bearing, carrying joists, a roof load, or a wall above, the header must be sized to span the opening and transfer that load to the trimmer studs and down to the foundation. A load-bearing header for a three-foot door is commonly a doubled 2×6 or 2×8 with a plywood spacer, but the correct size depends on the load and span. When in doubt, consult a building code table or an engineer. Cutting into a load-bearing wall without a properly sized header is a serious structural mistake and a code violation.

Tools and Materials Checklist

Framing an interior door rough opening cleanly calls for a modest kit:

• A reliable tape measure and a sharp pencil
• A 4-foot level to check plumb and a speed square for layout
• A circular saw or miter saw for cutting studs and headers
• A framing nailer or hammer and 16d nails
• Straight, dry 2×4 lumber; crowned or twisted studs throw the opening out of true
• Shims for the eventual door install

Select the straightest lumber you can for the king and trimmer studs. A single bowed trimmer will telegraph into the finished door as a gap or a bind, and no amount of shimming fully corrects a crooked jack stud.

Common Mistakes to Avoid

Beyond confusing slab and opening sizes, watch for these: framing the opening too tight leaves no shim room and forces the door out of plumb; framing it too loose wastes trim and leaves gaps the casing cannot cover; and forgetting the finished floor height leaves the door dragging or gapping. Take five minutes to double-check the math and your interior door rough opening will accept a prehung unit that swings true, latches clean, and needs nothing more than casing to finish.