RO dimensions from unit size, jamb width, and shim space
The rough opening (RO) is the framed hole a window or door installs into, and its whole purpose is to be slightly bigger than the unit — big enough to shim the unit level and plumb, small enough that the nailing fin or flange still lands on sheathing. Standard practice is the unit size plus ½ inch in width and ½ inch in height, giving roughly ¼ inch of shim space on every side, though manufacturer instructions always govern since some products specify different allowances.
This calculator takes unit width and height (the nominal size printed on the window, e.g., a '3050' window) plus a shim allowance and returns the rough opening, plus derived quantities framers actually cut: the header length, jack stud lengths, and sill/cripple counts for a full framing picture.
Reading window sizes is its own small skill worth spelling out. A '3060' window means 3'-0" wide by 6'-0" high nominal; a '3050' is 3'-0" × 5'-0". Doors use the same notation — an '0868' door is 2'-8" × 6'-8". But nominal is not glass or exact frame: always pull the actual unit dimensions and the manufacturer's RO spec from the order sheet before cutting lumber, because the half-inch rule is a default, not a law.
RO width = unit width + shim allowance (default ½" total); RO height = unit height + ½". The framed opening is built from a header across the top (its length typically RO width plus bearing on the jack studs each side — commonly the header runs the RO width plus 3" total for 1½" of bearing on each jack, per common framing practice and code bearing minimums), two jack studs supporting it, a rough sill below (for windows) spanning the RO, and king studs flanking everything.
Height coordination matters with the finished floor. For doors, the RO height accounts for the finished floor thickness and the door threshold; for windows, the sill height above finished floor is set by design (egress bedrooms have maximum sill heights — the IRC caps emergency-escape window sills at 44 inches above the floor) and by alignment with existing trim lines on remodels.
The table shows the standard nominal sizes and their conventional rough openings using the ½-inch rule. If your manufacturer specifies a different allowance, add theirs instead — the arithmetic is identical.
One remodeling reality check: actual masonry and old-frame openings in existing houses are never the size the tape says after decades of paint and settling. Measure each opening at three widths and three heights, use the smallest, and shim the difference — never assume the '3060' in the catalog matches the hole in the wall.
| Nominal size | Unit (W × H) | Rough opening (½" rule) |
|---|---|---|
| 2030 | 2'-0" × 3'-0" | 24½" × 36½" |
| 3050 | 3'-0" × 5'-0" | 36½" × 60½" |
| 3060 | 3'-0" × 6'-0" | 36½" × 72½" |
| 4040 | 4'-0" × 4'-0" | 48½" × 48½" |
| 6060 slider | 6'-0" × 6'-0" | 72½" × 72½" |
| 3068 door | 3'-0" × 6'-8" | per door manufacturer spec |
The shim space is not waste; it is the working room that makes a weathertight install possible. Units get set on sill flashing, shimmed plumb and level at the sides, fastened through the fin or framing, and the perimeter is sealed with backer rod and sealant or membrane. Squeeze the gap to nothing and you lose the ability to correct an out-of-square frame; leave too much and the fastening lands in air or the trim reveal blows out.
Egress and code dimensions are worth flagging early: bedroom windows on escape routes must meet minimum clear-opening dimensions (5.7 sq ft clear opening, at least 24" high and 20" wide clear, with sill no higher than 44" per the IRC's emergency escape rules) — which can force a bigger unit than the room's aesthetic called for. Checking this before ordering beats discovering it at inspection.
What is the rule for rough opening size?
Rough opening = unit size plus ½ inch in each dimension (about ¼" shim space per side) is the traditional default. Always confirm against the manufacturer's installation sheet — some units specify different allowances.
What does a 3050 window mean?
Nominal 3 feet 0 inches wide by 5 feet 0 inches high. First two digits are feet of width, second pair is feet and inches of height: 3060 is 3'-0" × 6'-0". Rough opening at the ½" rule is 36½" × 60½".
How much bearing does a window header need?
Commonly 1½ inches on each jack stud (the thickness of a 2x). Header size itself depends on span and load — use your plans or code tables for anything beyond standard openings.
How tall can a bedroom window sill be?
For emergency escape and rescue (egress) windows in bedrooms, the IRC caps the clear sill height at 44 inches above the finished floor, with minimum clear openings of 5.7 sq ft, 24" high, and 20" wide.
Should I measure old openings or trust the nominal size?
Measure. Existing openings in remodels are rarely true after settling and paint; measure width and height at three points each, use the smallest, and verify against the actual unit dimensions before cutting.