What is the standard rough opening for a window?

The standard rough opening is the window unit width plus 2 inches and the height plus 2.5 inches. For a 36x48 window unit, the rough opening is 38 inches wide by 50.5 inches tall. Always check the manufacturer's installation instructions, as some windows specify slightly different clearances.

How do I measure a rough opening for an existing window replacement?

For a replacement window in an existing rough opening, measure the RO width (between the king studs) and the RO height (from the top of the sill to the underside of the header). Subtract 1/2 inch from each dimension to get the window unit size to order, giving you clearance to shim. If the existing RO is out of square, measure both diagonals — if they differ by more than 1/2 inch, the RO may need reframing.

How big should the header be for a 6-foot window opening?

For a 6-foot opening in a load-bearing wall, a doubled 2x10 header is the common specification. If the load above is heavy (second floor, roof load) or the span is at the upper end, a doubled 2x12 or an LVL header of equivalent strength is advisable. In a non-load-bearing wall, a doubled 2x6 is typically sufficient. Confirm with your local building code requirements and a structural engineer for critical applications.

What is the difference between the rough opening and the window unit size?

The window unit size (or nominal size) is the dimension of the window itself as manufactured. The rough opening is the framed hole in the wall, which is 2 inches wider and 2.5 inches taller than the unit size to allow for shimming and the window flanges. When ordering a window, you order by the unit size, not the rough opening size.

Window Rough Opening Size Chart and Framing Guide

The standard rough opening for a window is the window unit width plus 2 inches and the height plus 2.5 inches — but the exact dimensions depend on the window manufacturer and your local framing practice. Here is the complete framing guide, including header sizing.

What Is a Rough Opening and How Is It Calculated?

A rough opening (RO) is the framed hole in the wall into which a window or door unit is installed. The RO is larger than the window's nominal or unit size to allow for shimming, leveling, and the window's nailing flanges. The standard rule of thumb for windows is: rough opening width = window unit width + 2 inches, and rough opening height = window unit height + 2.5 inches. So a 36-inch wide by 48-inch tall window unit requires a rough opening of 38 inches wide by 50.5 inches tall.

The extra 2 inches in width (1 inch on each side) allows for shimming the window plumb and level and provides clearance for the nailing flanges. The extra 2.5 inches in height accounts for shimming under the sill plate and clearance at the top. Some window manufacturers specify different tolerances — always check the installation instructions for the specific window you are installing, because premium windows with integral fins or between-the-studs installation may have slightly different requirements.

The ProBuildCalc window rough opening calculator lets you input the window unit size and it outputs the framing dimensions — RO width, RO height, jack stud height, sill plate height, and header size — so you can lay out the wall before cutting studs.

Framing Components: King Studs, Jack Studs, and Sill

A framed window opening has four key structural members beyond the regular stud layout. The king studs are full-height studs on each side of the opening that run from bottom plate to top plate. The jack studs (also called trimmer studs) are shorter studs that sit inside the king studs and support the header. The header spans the top of the rough opening and carries the load from above. The rough sill is a horizontal member at the bottom of the opening, and cripple studs fill the space between the rough sill and the bottom plate.

Jack stud height is the rough opening height plus the sill height from the floor, minus the header depth. For a window with an 8-inch header (common for wider windows in load-bearing walls), the jack stud height for a 50.5-inch RO at a sill height of 36 inches from the subfloor would be 36 + 50.5 = 86.5 inches from subfloor to top of RO, minus the header depth of 8 inches = 78.5 inches for the jack stud. This is the calculation that trips up most first-timers — the rough opening height on the plans is measured from the sill to the underside of the header, not from the floor.

Free tool: Rough Opening Calculator →

Header Size by Span

The header carries the load above the window and must be sized for the span (the rough opening width) and whether the wall is load-bearing. In a non-load-bearing wall, a single 2x4 or doubled 2x4 on the flat is often sufficient. In a load-bearing wall, header size is governed by the span and the load above. Common rules of thumb for load-bearing headers (confirm with local code and an engineer for your specific situation): up to 3-foot span use doubled 2x4; 3 to 5 feet use doubled 2x6; 5 to 7 feet use doubled 2x8; 7 to 9 feet use doubled 2x10; 9 to 11 feet use doubled 2x12.

LVL (laminated veneer lumber) headers are stronger than dimensional lumber of the same depth, so they can carry the same load in a shallower member. This matters when ceiling or floor structure above the window limits how deep the header can be. Engineered wood product (EWP) suppliers publish span tables for their LVL products — provide the span and load conditions and they will specify the right LVL size. For complex load situations (point loads, multi-story construction, large openings), consult a structural engineer — the cost of an engineering review is minor compared to the consequences of an undersized header.

Common Rough Opening Sizes for Standard Windows

Standard double-hung windows come in nominal sizes based on width and height in inches, coded as, for example, a 3040 window (30 inches wide by 40 inches tall in unit dimensions). Standard residential window widths run 24, 28, 30, 32, 36, 40, 44, 48, 60, and 72 inches. Heights commonly run 36, 42, 48, 52, 54, 60, 62, and 72 inches. Using the +2/+2.5 rule: a 3040 window (30x40 unit) needs a 32x42.5 rough opening; a 4060 window (40x60 unit) needs a 42x62.5 rough opening.

For egress windows in bedrooms — which must meet minimum opening area requirements of typically 5.7 square feet with a minimum width of 20 inches and minimum height of 24 inches — plan the rough opening around a compliant window unit first, then calculate the RO from the unit size. Egress requirements vary by code edition and jurisdiction, so confirm the requirements with your local building department before framing egress windows.

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FAQ

What is the standard rough opening for a window?: The standard rough opening is the window unit width plus 2 inches and the height plus 2.5 inches. For a 36x48 window unit, the rough opening is 38 inches wide by 50.5 inches tall. Always check the manufacturer's installation instructions, as some windows specify slightly different clearances.
How do I measure a rough opening for an existing window replacement?: For a replacement window in an existing rough opening, measure the RO width (between the king studs) and the RO height (from the top of the sill to the underside of the header). Subtract 1/2 inch from each dimension to get the window unit size to order, giving you clearance to shim. If the existing RO is out of square, measure both diagonals — if they differ by more than 1/2 inch, the RO may need reframing.
How big should the header be for a 6-foot window opening?: For a 6-foot opening in a load-bearing wall, a doubled 2x10 header is the common specification. If the load above is heavy (second floor, roof load) or the span is at the upper end, a doubled 2x12 or an LVL header of equivalent strength is advisable. In a non-load-bearing wall, a doubled 2x6 is typically sufficient. Confirm with your local building code requirements and a structural engineer for critical applications.
What is the difference between the rough opening and the window unit size?: The window unit size (or nominal size) is the dimension of the window itself as manufactured. The rough opening is the framed hole in the wall, which is 2 inches wider and 2.5 inches taller than the unit size to allow for shimming and the window flanges. When ordering a window, you order by the unit size, not the rough opening size.