What rough opening does a door or window need?
Pick the maker, enter the unit size, get the opening to frame — or the unit that fits an opening.
Enter the door size, such as 32 × 80.
Measure the framed opening between studs and from the floor or sill to the header; use the smallest numbers.
Type fractions as decimals: 35 1/2 is 35.5.
Use the numbers in your unit's own installation instructions.
Full-height studs at each end of the header, from IRC 2021 Table R602.7.5.
How big should the rough opening be for a door?
It depends on which size you start from, the nominal door, the frame or the whole unit, so the published rules differ. The Home Depot's interior prehung sheet asks for an opening 2 inches wider and 2½ inches taller than the door, so a 36 × 80 door takes 38 × 82½. JELD-WEN's interior prehung instructions (JII109) want the door frame ½ inch smaller than the opening both ways. JELD-WEN's exterior instructions (JII106) want the door 1 inch narrower and ¾ inch shorter than the opening, or ½ inch each where code requires, and 1¼ and ⅝ inch for 90-minute fire doors. Masonite's entry door sheet gives 38¼ × 82 for a 36 × 80 door that opens in, and 81⅛ high for one that opens out. Use the instructions that come with your door.
What rough opening does a window need?
Each maker states its own allowance over the frame size. Andersen's 400 Series Tilt-Wash double-hung needs a minimum opening of the window width plus ½ inch and the window height plus 0; Andersen adds that the minimum may need to grow to allow for building wraps, flashing, sill panning, brackets and fasteners. Marvin's Ultimate wood double hung asks for the frame size plus 1 inch wide and ½ inch tall, so a 21⅜ × 33 inch frame needs 22⅜ × 33½. Pella gives a clearance per side instead: ⅜ inch (¾ inch in all) for aluminum-clad wood and at least ¼ inch (½ inch in all) for Impervia fiberglass and vinyl windows. Always check the window's own installation instructions.
What about patio and sliding doors?
Andersen's 400 Series Frenchwood gliding patio door needs a minimum opening of the door width plus ¾ inch and the height plus ½ inch (¾ inch for four-panel doors), so a 72 × 80 two-panel door needs 72¾ × 80½. JELD-WEN's W-2500 wood-clad patio door is always ½ inch over the frame size. Pella sizes a door's width like a window and its height ½ to 1 inch greater than the frame (its 2026 design manual says ½ inch), with no clearance at the sill apart from any pan or flashing. The calculator shows each allowance with its source.
How do I frame around the opening?
The rough opening width is the header span. For exterior walls, the 2021 International Residential Code sets the minimum number of full-height studs at each end of a header in Table R602.7.5: for wind under 140 mph in Exposure B (or under 130 mph in Exposure C) one stud for a header up to 4 ft, two up to 8 ft, three up to 14 ft and four up to 18 ft; for wind up to 115 mph in Exposure B, one stud up to 8 ft and two up to 18 ft. A header between table values uses the number for the next longer span. Jack studs and the header size come from Table R602.7(1), which depends on span, snow load, building width and what the header carries, so use it or an engineer's design. We checked the 2021 edition only; use the edition your building department has adopted.
What if the opening is already framed?
Measure the width in three places and the height on both sides, and use the smallest numbers. Then pick “Unit that fits” to subtract the allowance and get the largest unit the opening can take; units come in set sizes, so choose the largest one at or under that size. Masonite's sheet notes that it is easier to reduce a rough opening with shims or an extra stud than to enlarge it. For a bedroom window, the opening is not the same as the clear opening that egress rules measure; see the egress window calculator.
Sources, read October 7, 2026: The Home Depot, “Installation Instructions: Interior Prehung”; JELD-WEN, installation instructions JII106 (May 2022) and JII109, and W-2500 Wood-Clad Patio Door Architectural Design Manual (March 2018); Masonite, “Installation Instructions for Prehung Entry Doors” (printed 12/08); Pella, “Rough Opening Clearance Recommendations” (Steady Set, 2024) and 2026 Architectural Design Manual; Andersen, 2025-26 400 Series Product Guide; Marvin, Ultimate Wood Double Hung sizing sheet (February 2020) and Ultimate Wood Double Hung Magnum manual (ver. 2021.4); International Residential Code 2021, Table R602.7.5 (ICC, codes.iccsafe.org). Bedroom windows: egress window calculator; the wall around the opening: framing calculator; the stairs to the door: stair calculator. How we check our calculators.