Rough-opening calculator

Size the framed hole for a window: add a shim gap to the unit dimensions so you can set it plumb, level and square.

Measure each opening and confirm sizes and clearances against the exact product you buy. Take three width and three height measurements and use the smallest of each; allow extra for custom sizes and waste. Sizes, clearances and rough-opening allowances vary by product and brand — read the spec sheet and the manufacturer’s data.
Your result
Rough opening36.50 in × 60.50 in
Window unit size35.50 in × 59.50 in
Shim gap (per side)0.50 in → +1.00 in each dimension

A 35.5 × 59.5 in window unit wants a rough opening of about 36.5 × 60.5 in with a 0.50 in shim per side. The rough opening is the framed hole — a bit larger than the window so you can shim it plumb, level and square; ½ inch per side is the common allowance, but confirm the exact rough-opening size on the manufacturer’s spec sheet before you frame.

Calculator inputs

in
in
in
Typical ½ in per side — confirm the exact figure on the manufacturer spec sheet

The rough opening (RO) is the framed hole in the wall — deliberately a little bigger than the window unit so you can shim it plumb, level and square and still have room for insulation and flashing. The standard allowance is about ½ inch of shim gap per side, which adds roughly 1 inch to both the width and the height of the unit.

That ½ inch is a convention, not a law: some manufacturers specify a different gap, and replacement (insert) windows have their own deduction. Always cross-check the exact RO on the product’s spec sheet before you frame — an opening framed too tight can’t be corrected without re-cutting.

Formula

rough_opening_width = unit_width + 2 × shim_gap
rough_opening_height = unit_height + 2 × shim_gap

With the default ½ in gap, that is the unit size + 1 inch in each direction.

Worked example

For a 35.5 × 59.5 inch window unit with a ½ inch shim gap per side:

RO width = 35.5 + (2 × 0.5) = 36.5 in
RO height = 59.5 + (2 × 0.5) = 60.5 in

So you would frame a 36½ × 60½ inch opening for that unit.

What to measure first & common mistakes

  • Confirm the manufacturer gap. ½ in per side is typical, but the spec sheet governs. Enter the specified gap if it differs.
  • New construction vs insert. This RO is for a new-construction / full-frame unit with a nailing fin. An insert (pocket) window fits inside the existing frame and uses a different deduction — measure the existing frame, not the RO.
  • Check for square. Measure both diagonals of the framed opening; they should match. A racked opening won’t let the window sit square even with the right RO.
  • Doors are different. A pre-hung exterior door adds about 2 in of width and 2½ in of height for the jamb and shims — use the door rough-opening calculator.

Reference table

Standard rough-opening allowances (labeled planning typicals — the spec sheet is the final word).

UnitAllowanceAdds to the unit size
Window (nailing-fin / full-frame)+0.50 in per side+1.00 in width & +1.00 in height
Pre-hung exterior doorjamb + shims+2.0 in width & +2.5 in height

Frequently asked questions

What is a rough opening for a window?
The rough opening is the framed hole in the wall, sized slightly larger than the window unit so you can shim it plumb, level and square. Typically it is the unit size plus about ½ inch of shim gap per side.
How much bigger should the rough opening be than the window?
A common allowance is ½ inch per side, which makes the opening about 1 inch wider and 1 inch taller than the window unit. Always confirm the exact figure on the manufacturer’s spec sheet.
Is the rough opening the same as the window size?
No. The rough opening is larger than the window unit by the shim gap on each side. Order the window to the unit size; frame the wall to the rough opening.
Does a replacement (insert) window use the same rough opening?
No. An insert window fits inside the existing frame, so you measure the existing frame and apply the manufacturer’s replacement deduction rather than framing a new rough opening.

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