Door Rough Opening Calculator

Introduction: what a door rough opening includes

A door rough opening is the framed hole a carpenter prepares in a wall before the prehung door, jamb, shims, and casing go in. It is deliberately larger than the door slab itself, because the installer needs room to stand the jamb plumb and square inside imperfect framing, slide tapered shims behind the hinge and latch sides, and leave clearance under the slab for flooring, a threshold, or an undercut for return air. Frame the hole exactly the size of the door and the unit physically will not fit; frame it far too large and the shims cannot bridge the gap.

For most residential prehung interior doors, a practical planning allowance is about 2 inches wider and 2 to 2.5 inches taller than the nominal slab size. Exterior doors, fire-rated doors, pocket doors, thick custom jambs, and specialty thresholds all shift those numbers, which is why this calculator lets you set the total extra width and height yourself and treats the manufacturer's installation sheet as the final authority.

Rough opening is a framing dimension, not a finished one. It is the clear distance measured between the inside faces of the two jack studs, and from the floor reference you will actually build to up to the underside of the header. Everything you can see once the room is finished, including drywall, casing, and the door stop, sits inside that hole rather than defining it. That distinction matters because a wall can look right and still be framed wrong: 1/2-inch drywall wrapped into an opening steals a full inch of width that the jamb was counting on.

The rough opening formula and its allowances

The math is intentionally transparent. For a prehung unit the rough opening is the slab dimension plus a total allowance in each direction:

The allowances are total opening allowances, not per-side values. Entering 2 inches of extra width leaves about 1 inch of clearance on each side of the jamb before shimming:

A pocket door does not follow that pattern at all, because the slab has to slide completely inside the wall. Frame kits are dimensioned from the slab width instead:

Once the opening is fixed, the header that caps it has to reach past the jack studs to bear on the king studs, so its length depends on how many jacks each side carries:

Plain-text formula: roughWidthIn = doorWidthIn + totalExtraWidthIn; roughHeightIn = doorHeightIn + totalExtraHeightIn + finishedFloorBuildupIn; sideClearanceIn = totalExtraWidthIn ÷ 2; pocketRoughWidthIn = 2 × doorWidthIn + 1; headerLengthIn = roughWidthIn + 2 × jacksPerSide × 1.5 in. Default planning allowance is door width + 2 in and door height + 2 to 2.5 in; the door manufacturer instructions control the final dimensions.

Framing anatomy: king studs, jack studs, and the header

The rough opening lives inside a small structural system. Full-height king studs run plate to plate on each side. Shorter jack studs (also called trimmers) are nailed to the kings and carry the header, the horizontal member that bridges the opening and picks up any load from above. Short cripple studs fill the space between the header and the top plate so the drywall has backing. The rough opening width is the clear distance between the jack studs; the rough opening height runs from the floor reference to the underside of the header.

Because dimensional lumber is 1.5 inches thick, the header must be longer than the opening it spans: with a single jack stud on each side the header length is the rough opening width plus 3 inches, and with doubled jacks (common for wide or heavily loaded openings) it is the rough opening width plus 6 inches. Measured across the outside faces of the king studs, the whole assembly occupies the header length plus another 3 inches, which is the number to use when you are laying out plates and checking that the opening still clears the next stud bay. The calculator prints these framing helpers with every result so you can rough out a cut list from the same numbers.

Where the 2-inch rule comes from

The common interior allowance is not folklore; it falls straight out of the jamb stock. A typical prehung interior jamb is 3/4-inch material, so the two jamb legs add 1.5 inches to the slab width. Add 2 inches total and you are left with roughly 1/4 inch of shim space per side, which is enough to plumb the jamb inside slightly bowed studs without exceeding what shims and casing can hide. Height works the same way: a 3/4-inch head jamb, roughly 3/4 inch of clearance under the slab for flooring, and a little shim space at the top add up to the familiar 2 to 2.5 inches over an 80-inch slab, which is why an 80-inch door usually gets an opening of 82 to 82.5 inches.

Manufacturers tend to state the same idea one step further along the chain. Therma-Tru's prehung installation instructions, for example, ask for a rough opening about 1/2 inch taller than the assembled frame and 1/2 to 3/4 inch wider, and because that frame already wraps the slab in jamb stock, the arithmetic lands back on roughly slab plus 2 inches. Working from the unit dimension is more precise when you have the unit in front of you; working from the slab size is more useful when you are framing before the door has been ordered.

Interior, exterior, and double doors

Exterior prehung units usually need a little more height than interior ones because the slab sits on a raised threshold over a sill pan, and the pan flashing and sloped sill eat vertical space. Plan on the same 2 inches of width and 2.5 inches of height as a starting point, then add another 1/2 inch of height if the unit ships with an adjustable sill or a separate sill pan. Double doors (a pair of slabs meeting with an astragal) follow the same width formula, just applied to the combined slab width: a pair of 30-inch slabs is 60 inches of door, so about a 62-inch rough opening.

Width matters for more than fit. The IRC requires at least one side-hinged egress door per dwelling unit that gives a clear width of not less than 32 inches with the door open 90 degrees and a clear height of not less than 78 inches, measured to the face of the door and the stop rather than to the framing. A 32-inch slab does not deliver 32 inches of clear width once the jamb, stop, and open door leaf are subtracted, so a 36-inch door is the usual way to satisfy that requirement comfortably.

Pocket, bifold, and bypass openings

Specialty doors break the slab-plus-allowance model in different directions, and mixing them up is the single most expensive framing mistake on this page.

Pocket doors need roughly double the slab width plus about an inch, because the split-stud frame has to swallow the entire slab. Johnson Hardware's 1500 series frames, one of the most widely stocked kits in North America, list a 61-inch by 84.5-inch rough opening for a 30-inch door and a 65-inch by 84.5-inch opening in a 2x4 wall for a 32-inch door. That is 2 x 30 + 1 and 2 x 32 + 1 respectively, with 4.5 inches added to the 80-inch slab height for the track.

Bifold doors are sized to the finished opening rather than to a rough opening. Reeb's supplier documentation is explicit: bifold finished opening widths are true to the call size, so a 3/0 bifold wants a 36-inch finished opening and a 6/0 unit wants 72 inches, with the slabs themselves running about 1/4 inch undersized so the hardware has room. The standard finished opening height is 81 inches. To get from there to a rough opening you add the jamb stock back on, which is why a 3/0 bifold is normally framed at about 38 inches wide and 82 inches tall.

Bypass (sliding closet) doors behave like bifolds: the track is cut to the finished opening, and the panels overlap. Barn-door hardware is different again, because the door hangs on the face of the wall and the opening only has to be smaller than the slab, not larger. Use the slab-only or custom mode for all of these and let the hardware instructions govern.

Header size and jack studs under the IRC

This calculator sizes the length of the header, never its depth. Depth is a structural question, and the International Residential Code splits it cleanly in two.

In an interior or exterior nonbearing wall, section R602.7.4 permits a single flat 2-inch by 4-inch member to serve as the header for openings up to 8 feet wide, provided the vertical distance to the parallel nailing surface above is not more than 24 inches; no cripples or blocking are required above such a header. That covers the overwhelming majority of interior partition doors, which is why so many remodels get away with a flat 2x4 on edge-nailed jacks.

In a load-bearing wall, the header must be sized from IRC Table R602.7(1) for exterior bearing walls or Table R602.7(2) for interior bearing walls, using your building width, ground snow load, and the number of floors and roofs the wall supports. For a 28-foot-wide building with a 30 psf ground snow load carrying one floor plus the roof, the published spans land near a doubled 2x6 for a 4-foot opening, a doubled 2x10 for a 6-foot opening, and a doubled 2x12 for an 8-foot opening; beyond that the header needs engineering. Headers must bear on at least 1.5 inches of jack stud at each end, and Table R602.7.5 sets how many full-height studs each end needs as the span and wind exposure grow.

Worked example: framing a 32 by 80 bedroom door

Suppose you are hanging a 32 inch by 80 inch interior prehung door with 2 inches of width allowance and 2.5 inches of height allowance, in a nonbearing partition, on a subfloor that will later receive 3/4 inch of engineered hardwood.

Width: 32 + 2 = 34 inches

Height from the finished floor: 80 + 2.5 = 82.5 inches

Height from the bare subfloor: 80 + 2.5 + 0.75 = 83.25 inches

Header length, one jack per side: 34 + 3 = 37 inches

King stud outside-to-outside: 37 + 3 = 40 inches

That gives a planning rough opening of 34 in wide by 83.25 in tall from the subfloor, with about 1 inch of clearance per side before shimming and a header cut at 37 inches. Because the wall is nonbearing and the opening is well under 8 feet, a single flat 2x4 header is code-compliant here. Before cutting or framing, compare those numbers against the sheet packed with the actual door.

Common planning sizes

Specialty openings compared with a plain prehung door

How to use this rough opening calculator

1. Select the planning mode that matches the door package: interior prehung, exterior prehung, double pair, pocket, bifold, slab-only note, or custom manufacturer allowance. Each mode preloads a sensible starting allowance, and pocket mode switches to the doubled-width frame rule automatically.
2. Choose a common slab size, including double-door pairs, or enter the actual door width and height in inches. A 30-by-80 door usually means a 30 inch wide, 80 inch tall slab.
3. Enter the total extra width and total extra height from the door instructions or your own planning allowance. These are total opening allowances, not per-side values.
4. If the finished floor is not down yet, enter its build-up thickness so the height is measured correctly from the bare subfloor. Leave it at zero when you are measuring from the surface the jamb will actually sit on.
5. Tell the calculator whether the wall is bearing or nonbearing and how many jack studs each side will carry, so the header length and the code note match what you are about to build.
6. Read the result in decimal inches, carpenter fractions, feet and inches, and millimeters, then compare everything with the manufacturer instructions before framing, cutting, or ordering material.

If the door project is part of a larger remodel, the drywall sheet calculator can net the opening out of the wall area, and the baseboard and trim calculator covers the casing and base runs that finish the room.

Assumptions and limitations of these opening figures

• Planning aid, not a framing specification. It does not size headers structurally, evaluate load-bearing walls, or decide fire-rating clearances. Header depth on a bearing wall is governed by IRC span tables and by the authority having jurisdiction.
• Standard lumber assumed. The header-length helper assumes 1.5-inch-thick dimensional studs; engineered lumber, steel framing, and 2x6 walls change the arithmetic and the pocket-frame sizes.
• Flooring is your input. The build-up field only knows what you type. It cannot see whether tile, underlayment, or carpet is coming, and it does not add an undercut for return air.
• Specialty doors are approximations. Pocket, bifold, bypass, and barn-door hardware have their own opening rules that override the slab-plus-allowance model, and kits differ between manufacturers and between 2x4 and 2x6 walls.
• Egress and accessibility are separate checks. Clear width is measured to the face of the open door and the stop, not to the framing, so meeting a rough opening number does not by itself satisfy IRC R311.2 or an accessibility standard.
• Manufacturer sheet wins. Confirm final dimensions with the product instructions, local code requirements, and a qualified carpenter or inspector whenever the wall is structural.

Sources checked for these rules

Sources. The header, jack stud, and egress rules on this page were checked against the model code text and against published manufacturer installation data rather than trade rules of thumb:

• ICC Digital Codes, 2021 International Residential Code, section R602.7 Headers - header span tables R602.7(1) and R602.7(2), and jack stud counts in Table R602.7.5.
• UpCodes, IRC nonbearing wall headers (R602.7.3 / R602.7.4) - the single flat 2x4 header allowance for openings up to 8 feet.
• UpCodes, IRC R311.2 Egress door - 32 inch minimum clear width and 78 inch minimum clear height for the required egress door.
• Johnson Hardware, 1500 Series pocket door frame kits - published rough opening sizes behind the 2 x slab width + 1 inch rule.
• Reeb Learning Center, bifold finished openings - bifold call sizes are finished opening widths, with a standard 81 inch finished opening height.
• Therma-Tru, prehung entry and patio door installation instructions (PDF) - rough opening sized from the assembled frame plus shim clearance, with sill pan guidance.
• Masonite, how to measure for interior doors - jamb widths of 4-9/16, 5-1/4, and 6-9/16 inches for common wall thicknesses.

Door rough opening: frequently asked questions