Standard Rough Opening Formulas
The rough opening (RO) must be slightly larger than the window unit to allow for shimming and leveling. The NAHB standard additions are:
RO Width = window unit width + 2″
RO Height = window unit height + 2.5″
Jack stud height = sill height from floor + RO height
King stud length = (ceiling height × 12) − 3″
Cripple stud height (below sill) = sill height − 1.5″
The 2″ width addition provides roughly 3/4″ of shimming clearance on each side. The 2.5″ height addition accounts for the rough sill plate (1.5″) and shim space at the top and bottom (0.5″ each). Always verify against the window manufacturer's installation guide, as some specify a different RO.
Common Window Sizes, Rough Openings & Headers
| Window Unit | RO Width | RO Height | Header (load-bearing) |
|---|---|---|---|
| 24" × 36" | 26" | 38.5" | Doubled 2×6 |
| 30" × 48" | 32" | 50.5" | Doubled 2×6 |
| 36" × 48" | 38" | 50.5" | Doubled 2×6 |
| 48" × 48" | 50" | 50.5" | Doubled 2×8 |
| 60" × 48" | 62" | 50.5" | Doubled 2×8 |
| 72" × 60" | 74" | 62.5" | Doubled 2×10 |
| 84" × 60" | 86" | 62.5" | Doubled 2×10 |
| 96" × 60" | 98" | 62.5" | Doubled 2×12 |
Header sizing above is for single-story or top-floor load-bearing walls under typical residential loads. Non-load-bearing walls may use smaller headers. Consult local codes or a structural engineer for multi-story or high-load applications.
Egress Window Requirements
Bedroom windows below grade (basement egress) must meet minimum egress requirements per IRC Section R310. The key minimums are:
- Net clear opening area: 5.7 sq ft minimum (5.0 sq ft for ground floor)
- Minimum net clear height: 24 inches
- Minimum net clear width: 20 inches
- Maximum sill height: 44 inches above the floor
A common code-compliant egress window is 36″ wide × 24″ tall (net clear), which provides 6.0 sq ft of clear opening. The rough opening would be 38″ × 26.5″. Note that egress windows always require a building permit.
Frequently Asked Questions
What is a rough opening for a window?+
A rough opening is the framed hole in a wall that a window fits into before trim and finishing. It is intentionally larger than the window unit to allow for shimming, leveling, and squaring the window during installation. The rough opening is bounded by a header at the top, a rough sill at the bottom, and jack (trimmer) studs on the sides.
How much bigger is a rough opening than the window?+
The standard rough opening is 2 inches wider and 2.5 inches taller than the window unit dimensions. The extra width allows for 3/4" of shimming on each side plus clearance. The extra height accounts for the rough sill thickness (1.5") and shim space top and bottom (0.5" each). Some window manufacturers specify their own required rough opening — always check the spec sheet.
What size header do I need for a window?+
Header size is determined by the rough opening width. A common guide for load-bearing walls: RO ≤ 42" → doubled 2×6; 43"–66" → doubled 2×8; 67"–90" → doubled 2×10; 91"–120" → doubled 2×12; over 120" → LVL beam. Non-load-bearing walls can use smaller headers. Always verify with local building codes.
Do I need a building permit to replace or add a window?+
Replacing a window in the same rough opening generally does not require a permit in most US jurisdictions. However, cutting a new window opening, enlarging an existing opening, or adding a window in a load-bearing wall almost always requires a permit and possibly engineer approval. Check with your local building department before starting work.