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Rough Opening Calculator

Enter the nominal door or window size to get the correct rough opening dimensions and a header size guide.

Opening

Enter the door or window unit size (the size on the label). The rough opening is cut larger for framing and shimming.

Rough opening

Width × Height
2" × 2.5"
enter dimensions
Header guide4×4 or (2) 2×4

Header sizing depends on load, species, and local code — always verify with span tables. Double check RO against manufacturer specs before framing.

How to find a rough opening size

For a pre-hung door: add 2 inches to the width and 2.5 inches to the height. For a window: add 1 inch to the width and 1/2 inch to the height. Always verify against the manufacturer's installation guide — actual clearances vary by product.

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Rough opening FAQ

What is a rough opening?
A rough opening is the framed hole in a wall — wider and taller than the actual door or window unit — that provides space for the frame, shimming, and installation. The door or window unit sits inside and is shimmed plumb and level.
How big is a rough opening for a door?
A standard rough opening for a pre-hung door is the nominal door width plus 2 inches wide (for a jack stud on each side) and the door height plus 2.5 inches (for the sill plate, door frame, and shimming). A 36×80-inch door needs a 38×82.5-inch rough opening.
How big is a rough opening for a window?
Windows typically use a rough opening that is 1 inch wider and 1/2 inch taller than the window unit. Always check the window manufacturer's installation specs — some require different clearances.
What size header do I need?
Header size depends on the span, load above, lumber species, and local code. As a rough guide: spans up to 36 inches often use doubled 2×4 or 2×6; wider spans step up from there. The calculator gives a starting point, but confirm with span tables or an engineer for load-bearing walls.

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