Door Rough Opening: Size It From the Actual Unit

For a conventional prehung door, the rough opening is the framed opening between the side framing and from the specified floor reference to the underside of the header. It must be larger than the outside dimensions of the complete door unit so the installer has room to square, plumb, shim and fasten the jamb.

The dependable calculation is:

Rough-opening width = actual unit width + required installation clearance Rough-opening height = actual unit height + required installation clearance

Use the door schedule or manufacturer’s submittal for that clearance. A common requirement is 1/2 inch total extra width and 1/2 inch total extra height—about 1/4 inch at each side—but it is not universal. Andersen specifies that allowance for one wood entry-door system, while an interior prehung-door guide likewise requires the frame to be 1/2 inch smaller than the rough opening in both directions (Andersen installation guide; ReliaBilt interior-door instructions).

Do not order or frame from the nominal slab size alone. The jamb, head, threshold, astragal, mullions and sidelites can all change the unit dimensions.

Common planning sizes for a prehung swing door

Before a unit has been selected, residential framing is often laid out using this rule of thumb for a typical interior prehung door:

Nominal slab width + 2 inches Nominal slab height + 2 1/2 inches

That produces these preliminary dimensions for common 80-inch doors:

Nominal door slab	Preliminary rough opening
24 × 80 in.	26 × 82 1/2 in.
28 × 80 in.	30 × 82 1/2 in.
30 × 80 in.	32 × 82 1/2 in.
32 × 80 in.	34 × 82 1/2 in.
36 × 80 in.	38 × 82 1/2 in.

Treat the table as layout guidance, not a purchase specification. It assumes a typical assembly whose outside frame is about 1 1/2 inches wider and 2 inches taller than the nominal slab. Actual products vary. Masonite, for example, identifies 34 1/2- and 38 1/2-inch widths as common exterior rough openings and explicitly warns that sizes vary (Masonite measuring guide).

Once the exact product is known, replace the rule of thumb with its listed unit and rough-opening dimensions. This is especially important for exterior systems, double doors, transoms, sidelites, fire-rated openings and commercial hollow-metal frames.

Measure at the framing, not the trim

For replacement work, remove enough casing to expose the structural opening. Existing trim dimensions or the visible space between jambs do not establish the rough opening.

1. Measure width at the top, middle and bottom, stud to stud. Record the smallest measurement.
2. Measure height at both sides, using the floor reference required by the door instructions. Record the smaller measurement.
3. Check the side framing for plumb and the sill or floor line for level.
4. Compare both diagonals to check square.
5. Check the wall plane for twist, using a straightedge, level or crossed strings.
6. Measure jamb depth separately so the jamb suits the finished wall thickness.

Masonite directs replacement-door measurers to take several stud-to-stud width readings and use the smallest, then measure from the subfloor to the opening head on both sides (Masonite measuring guide). Product tolerances still control: the Andersen guide cited above requires its opening to be plumb and level, with diagonals within 1/8 inch. The ReliaBilt interior guide allows up to 1/4 inch difference between diagonals but limits plumb and level deviation to 1/8 inch.

An opening can have the correct average dimensions and still reject the unit at one corner. It can also accept the frame but leave too little shim space to correct a bowed or leaning trimmer stud.

Establish the floor reference before framing the height

“From the floor” is incomplete unless the crew knows which floor:

• Subfloor: a common reference for exterior units and many new-construction schedules.
• Finished floor: may control a pocket-door kit, retrofit or another product whose instructions use that datum.
• Threshold or sill elevation: affects exterior-door drainage, landings and accessible-route details.

Account for underlayment, tile, wood flooring, carpet and transitions before fixing the header height. Interior prehung jamb legs may extend below the slab and be trimmed for the floor condition; the ReliaBilt instructions specifically note this adjustment. Coordinate that elevation before nearby electrical, plumbing and HVAC rough-in work makes reframing costly.

The opening must be square enough to use the clearance

Installation clearance is not permission to leave the framing loose or crooked. The jamb needs support at its fastening and shim locations. Correct a crowned sill, bowed stud, out-of-plumb side or racked opening before setting the door.

Do not fill a substantially oversized opening with a thick stack of loose shims. Add properly fastened framing where needed while preserving the required structural header support. In a bearing wall, the header is not selected merely by door width: IRC header tables vary with supported roof and floor loads, building width, snow load, lumber arrangement and the required number of jack studs (2021 IRC Table R602.7(1)). Check the locally adopted code before altering a bearing or braced wall.

Pocket and bifold doors use different dimensions

The slab-plus-2-inch rule is for a typical hinged prehung door, not every door type.

A pocket-door rough opening accommodates both the passage and the wall pocket. For an 80-inch door, Johnson Hardware’s current 1500PF-series instructions specify a minimum width of twice the door width plus 1 inch and a minimum height of approximately 84 inches. The sheet also gives separate layout measurements from finished floor and subfloor (Johnson Hardware 1500PF instructions). Other pocket systems can differ.

A bifold is commonly specified by its finished opening, after drywall or wood jambs—not simply by a framed rough opening. One Masonite instruction sheet calls for a 30 × 80 3/4-inch finished opening for its nominal 30-inch bifold with 79-inch panels (Masonite bifold instructions). Frame backward from the specified finished opening, adding the actual thickness of jamb material or drywall at both sides and the head.

Barn doors, bypass doors and cased openings likewise follow their hardware and finished-opening requirements rather than the prehung-door table.

Size for clear passage, not just the label

A nominal door width is not the same as clear passage width. The 2021 IRC requires at least one side-hinged egress door for each dwelling unit with a clear width of at least 32 inches, measured from the door face to the stop with the door open 90 degrees, and a clear height of at least 78 inches (IRC R311.2). The locally adopted edition and amendments govern the project.

Accessible routes involve more than a rough-opening number. The U.S. Access Board explains that clear width is measured to the open door face—or the leading edge of a sliding or folding door—and that maneuvering clearances depend on approach direction, swing, closer and latch conditions (Access Board door guide). Verify the complete assembly and surrounding floor space rather than assuming a nominal 32- or 36-inch slab establishes compliance.

The reliable field sequence is to select the exact assembly, verify clear-passage and code requirements, obtain the unit and rough-opening dimensions, establish the floor elevation, and only then frame and inspect the opening.