How to size windows
Rough opening vs. finished size
Window nominal size (e.g., 3050 = 3’6″ × 5’0″) is the marketing label. Rough opening is the framed hole in the wall, usually 1/2″ larger than the window unit. Window unit dimension is the frame size, the actual product. Glass / sash size is what daylight passes through. Each is different; never order from the wrong number.
Standard sizes by use
US standard window sizes are quoted in 4-digit codes: 2020 (24″×24″), 3030 (36″×36″), 4040 (48″×48″), and so on. Bedroom egress requires a clear opening of at least 5.7 sq ft, with a minimum width of 20″ and minimum height of 24″ (US IRC; verify locally). Sill height for egress: max 44″ from finished floor.
Code and egress
Bedrooms must have an egress window or door (size, sill height, and operability vary by jurisdiction). For doors, check the rough opening, the door's swing direction, and the trim allowance before ordering.
Common mistakes
- Confusing nominal and rough-opening dimensions; they differ by 2–2.5″ for most pre-hung doors.
- Forgetting handle and lock side — this affects which way the unit must be ordered.
- Skipping the egress check on a bedroom remodel.