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.
The rough opening formula
The math is intentionally transparent. 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:
Plain-text formula: roughWidthIn = doorWidthIn + totalExtraWidthIn; roughHeightIn = doorHeightIn + totalExtraHeightIn; sideClearanceIn = totalExtraWidthIn ÷ 2; headerLengthIn ≈ roughWidthIn + 3 with one jack stud per side. 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 subfloor 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. The calculator prints this framing helper 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.
Interior, exterior, and double doors
Exterior prehung units need 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; the exterior planning mode therefore starts at 3 inches of height allowance instead of 2.5. 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. Bifold and bypass closet doors are the exception to everything above, because they are sized to the finished opening rather than a framed rough opening, and pocket doors need roughly double the slab width to house the frame kit inside the wall. Use the slab-only and custom modes for those cases and let the hardware instructions govern.
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:
Width: 32 + 2 = 34 inches
Height: 80 + 2.5 = 82.5 inches
That gives a planning rough opening of 34 in wide by 82.5 in tall, with about 1 inch of clearance per side before shimming and a header cut at roughly 37 inches for single jacks. Before cutting or framing, compare those numbers against the sheet packed with the actual door, especially if the floor is unfinished or the opening needs a threshold.
Common planning sizes
| Door slab | Planning rough opening | Approximate metric | Common use |
|---|---|---|---|
| 24 in x 80 in | 26 in x 82.5 in | 660 x 2096 mm | Small closet or powder room |
| 28 in x 80 in | 30 in x 82.5 in | 762 x 2096 mm | Closet or small interior room |
| 30 in x 80 in | 32 in x 82.5 in | 813 x 2096 mm | Interior passage door |
| 32 in x 80 in | 34 in x 82.5 in | 864 x 2096 mm | Bedroom or wider passage door |
| 36 in x 80 in | 38 in x 82.5 in | 965 x 2096 mm | Entry, accessibility, or utility space |
| 36 in x 96 in | 38 in x 98.5 in | 965 x 2502 mm | Tall entry door |
| 60 in x 80 in (pair of 30s) | 62 in x 82.5 in | 1575 x 2096 mm | Double doors to a den or patio |
| 72 in x 80 in (pair of 36s) | 74 in x 82.5 in | 1880 x 2096 mm | Wide double-door opening |
How to use this rough opening calculator
- Select the planning mode that best matches the door package: interior prehung, exterior prehung, slab-only note, or custom manufacturer allowance. Interior and exterior modes preload a sensible starting allowance.
- 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.
- 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.
- Read the result in decimal inches, carpenter fractions, feet and inches, and millimeters, plus the header-length framing helper, 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
- Planning aid, not a framing specification. It does not size headers structurally, evaluate load-bearing walls, or decide fire-rating clearances. Header sizing on a bearing wall is an engineering question governed by span tables and local code.
- Standard lumber assumed. The header-length helper assumes 1.5-inch-thick dimensional studs; engineered or steel framing changes the arithmetic.
- Flooring is your input. The height allowance must include finished-floor buildup you know about; the tool cannot see whether tile, underlayment, or carpet is coming.
- Specialty doors excluded. Pocket, bifold, bypass, and barn-door hardware have their own opening rules that override the slab-plus-allowance model.
- Manufacturer sheet wins. Confirm final dimensions with the product instructions, local code requirements, and a qualified carpenter or inspector whenever the wall is structural.
Door rough opening: frequently asked questions
How much bigger should a rough opening be than the door?
A common starting point for a prehung interior door is about 2 inches wider and 2 to 2.5 inches taller than the nominal door slab. The exact allowance depends on the jamb, shimming space, flooring, threshold, and manufacturer instructions.
Is rough opening measured from framing or drywall?
Rough opening is measured between unfinished framing members: king or jack studs side to side, and subfloor or finished floor reference to the underside of the header. Trim and drywall are not part of the rough opening measurement.
What is the rough opening for a 36 inch exterior door?
A common plan for a 36 by 80 exterior prehung door is a rough opening of about 38 inches wide by 82.5 to 83 inches tall, because the sill pan and threshold usually need more height than an interior head jamb. Exterior units vary more than interior ones, so treat the installation sheet packed with the door as the final answer.
What if my rough opening is too big or too small?
An opening up to about an inch too wide can usually be saved with wider shims or by padding one side with a ripped strip of plywood or lumber nailed to the jack stud. An opening that is too small is the harder problem: jambs cannot be shimmed into space that does not exist, so a stud usually has to be moved or the header reset. That is why framers err slightly large, never small.
Do I measure rough opening height from the subfloor or the finished floor?
Measure from the surface the jamb legs will actually sit on. If flooring is not installed yet, measure from the subfloor and add the finished-floor thickness to your height allowance so the door clears carpet, tile, or hardwood that arrives later. If the finished floor is already down, measure from it directly.
Can this replace the door manufacturer's instructions?
No. Use this calculator for planning and sanity checks, then confirm the final opening with the specific prehung door, jamb kit, threshold, fire rating, and local framing requirements.
Arcade Mini-Game: Door Framing Layout Run
Catch useful framing inputs and avoid assumptions that throw off the rough opening.
Start the game, then use your pointer or arrow keys to catch useful inputs and avoid bad assumptions.