Window rough opening calculator

The framed opening to leave in a wall for a new-construction window, sized from the unit. Width, height, and the header length — with the shim gap tied to the frame material instead of hidden inside one fixed number.

01 How the math works

A new-construction window comes as a single unit with an integral nailing flange — no separate jamb for the framing to swallow. The carpenter frames an opening a little larger than the unit, sets it in, then shims it plumb, level, and square before nailing off the flange. The rough opening is the unit size plus a shim gap on every side.

The gap is the same at both jambs, at the head, and at the sill, so it lands on height exactly as it lands on width. A window is shimmed on four sides, not three — unlike a door, there is no floor to leave open at the bottom.

With a standard 1/4 in (6 mm) shim gap each side, the allowance is 2 × 1/4 = 1/2 in on both width and height. That is the familiar "unit plus half an inch" — written as a sum so you can see it is two shim gaps, not a constant, and swap the gap when the frame changes.

02 Worked example

Take the common bedroom window: a 36 in × 48 in double-hung unit (914 mm × 1,219 mm), vinyl-framed, with a 1/4 in (6 mm) shim gap on each side.

Width: 36 + 2 × 1/4 = 36-1/2 in (927 mm). Height: 48 + 2 × 1/4 = 48-1/2 in (1,232 mm). So the framed opening is 36-1/2 × 48-1/2 in. The header above it bears on two jack studs, so it spans the opening width plus two 1-1/2 in jacks: 36-1/2 + 3 = 39-1/2 in (1,003 mm). A fiberglass unit of the same size, framed at 3/8 in per side, needs a 36-3/4 × 48-3/4 in opening instead.

03 When this calculator is wrong

The number here is a per-side shim gap, not a universal constant. Two things break a fixed-addition "window rough opening" rule, and both have a number attached.

• The frame material changes the gap. Vinyl, wood, and aluminum units frame at about 1/4 in per side — a +1/2 in total. Composite and fiberglass frames, and mulled (coupled) multi-unit assemblies, commonly call for 3/8 in per side — a +3/4 in total, a quarter-inch more in each dimension. Set the shim to match the unit, and read the actual rough opening off its spec sheet when you can.
• Too big is a problem, not a safety margin. The shim gap exists to square the unit in a slightly-off opening and to hold low-expansion foam or backer rod. Past roughly 1/2 in of gap per side, shims alone no longer bridge it — the opening wants filler framing (a ripped plywood packer or an added stud) before the window goes in. On windows, "order it bigger to be safe" costs you re-framing, not peace of mind.
• It is not the door rule. A pre-hung door opening is the slab plus 2 in of jamb and shim; borrow that here and a 36 in window lands in a 38 in hole, over an inch too wide per side. A window has no pre-hung jamb to fill. The two openings are framed to different numbers for the same reason a 2×4 and a 2×6 are both 1-1/2 in thick — the name describes the part, not the gap around it.
• It sizes the opening, not the header. The header length here (opening width + 3 in) is the board length only. Its depth — 2×6 through 2×12 or built-up — is set by span and load in IRC Table R602.7(1), not by this calculator. A masonry opening is sized separately and runs larger.

04 What to do with the result

Frame the opening at the width and height above, then check it for square across both diagonals before the unit arrives — a 1/4 in of shim per side only trues a rough opening that started close. Confirm the exact rough opening on the window's own spec sheet, since the shim gap shifts with frame material and product line. For the header, use the length here and size its depth from the span table.

05 Common questions

What is the rough opening for a window?

For a standard vinyl, wood, or aluminum unit, frame the opening about 1/2 in larger than the window in both width and height — a 1/4 in (6 mm) shim gap at each jamb, the head, and the sill. Composite, fiberglass, and mulled units take 3/8 in per side, a 3/4 in total.

How much bigger should a rough opening be than the window?

A 1/4 in shim gap on each side is the common standard, so the opening is 1/2 in wider and 1/2 in taller than the unit. The gap lets you shim the window plumb and square and leaves room to seal with low-expansion foam. Confirm it against the unit's spec sheet.

What is the rough opening for a 36x48 window?

36-1/2 in wide × 48-1/2 in tall for a standard-frame unit at 1/4 in per side (927 × 1,232 mm). A fiberglass or composite unit of the same size wants 36-3/4 × 48-3/4 in.

Can a window rough opening be too big?

Yes. Past about 1/2 in of gap per side, shims no longer bridge the space cleanly, and the opening needs filler framing before the window is set. Oversizing a window opening makes more work, not a safer install.

Is a window rough opening the window size plus 2 inches?

No — that is the pre-hung door rule, where 2 in covers the jamb and shims. A new-construction window has no separate jamb, so its opening is only about 1/2 in larger than the unit. Borrowing the door number leaves a gap too wide to shim.