Framing a new addition or planning a window replacement order goes sideways fast when the rough opening does not match what the factory stocks. Typical window sizes follow industry conventions that every manufacturer — Andersen, Pella, Marvin, Jeld-Wen, Milgard — shares with only minor variation. Stick to those standard sizes and you pay catalog prices with a two-week lead time. Order custom, and you add 30-60 percent to the unit price plus another four to eight weeks of factory time. This guide covers the common dimensions, how window sizes are called out, and how to measure so the installer does not send the unit back.
How Window Sizes Are Written
Manufacturers use a four-digit code. The first two digits describe width in feet and inches; the last two describe height. A “2846” window is 2 feet 8 inches wide by 4 feet 6 inches tall. Some brands publish the actual unit dimension (the outside of the frame) and some publish the rough opening — always verify which before ordering. The rough opening is typically 1/2 to 3/4 inch larger in each direction than the unit dimension to allow for shimming.
European-built windows and custom shops often use metric callouts (millimeters wide by tall). Mixing metric and imperial on a plan set creates expensive mistakes.
Double-Hung Standards
Double-hung units are the most common residential window in the US. Widths run 24, 28, 32, 36, and 40 inches. Heights run 36, 44, 54, 62, and 72 inches. The sweet spot for a main-floor bedroom is a 3040 or 3046 — 36 inches wide by 46 inches tall, which satisfies egress code in most jurisdictions.
For bathrooms, undersized double-hungs at 24 by 36 (a 2436) or 28 by 42 (a 2842) balance privacy with ventilation. Stacking two double-hungs side by side is cheaper than a single wide window and meets most kitchen and living-room needs.
Casement and Awning Windows
Casement windows swing out on side hinges and usually come in widths of 18, 22, 26, 30, and 34 inches by heights of 30, 42, 54, and 66 inches. Because the full sash opens, a 2 by 4-foot casement delivers more ventilation than a double-hung of the same size. Code-compliant egress casements typically measure 24 by 48 or larger.
Awning windows hinge at the top and tilt outward from the bottom — good for rainy climates because they shed water when open. Stock widths are 24, 28, 32, 36, and 48 inches with heights of 20, 24, 30, and 36 inches. Awnings are common above kitchen sinks and at the tops of picture-window arrangements.
Sliders and Picture Windows
Horizontal sliders stock in widths of 36, 48, 60, and 72 inches with heights of 24, 36, 48, and 60 inches. A 6-foot by 4-foot slider is a classic master-bedroom egress choice in ranch-style homes. Sliders are simpler than double-hungs, so they cost 10-20 percent less at comparable quality tiers.
Picture windows — fixed, non-operable units — come in a wider range because they do not need to match egress or balance requirements. Common sizes run 24 to 96 inches wide by 24 to 72 inches tall. Large picture windows deliver dramatic views but gain and lose heat aggressively; spec at least a Low-E double-pane with argon fill for anything over 20 square feet.
Bay, Bow, and Specialty Shapes
Bay windows project outward from the wall as three units: a central picture window flanked by two double-hungs or casements at 30-45 degree angles. Standard widths run 3.5 to 10.5 feet and heights 3 to 6.5 feet. Bow windows use four to six equal-size units curving out in a gentle arc — widths typically 7 to 12 feet.
Specialty shapes include half-round transoms (commonly 30-72 inches wide), quarter-round accents, octagon and oval windows, and gable-end triangles. These are often priced and lead-timed as custom units regardless of how standard they look.
Egress Requirements
Any bedroom on any floor needs a window large enough to serve as an emergency escape. The International Residential Code requires a minimum net clear opening of 5.7 square feet on upper floors (5.0 square feet at grade), minimum clear height of 24 inches, minimum clear width of 20 inches, and a sill height no more than 44 inches above the finished floor.
That math eliminates many otherwise attractive standard sizes. A 2840 double-hung yields only about 4.4 square feet of clear opening — not enough. Stick to 3046 or larger for bedrooms and confirm the net opening, not the unit dimension.
Measuring for Replacement
For insert replacements (keeping the existing frame), measure the pocket in three places: at the top, middle, and bottom for width and at the left, middle, and right for height. Use the smallest measurement in each direction. Deduct 1/4 inch in each direction and that is the unit to order.
For full-frame replacements, remove the interior trim and measure the rough opening stud-to-stud and header-to-sill. Order a unit 1/2 inch smaller in each direction. Always measure at least two windows on the same wall — old homes rack and twist, and no two openings on the same wall are guaranteed identical.
Ordering and Lead Time
Stock sizes at a regional big-box or dealer typically ship in two to five business days if in inventory. Factory-built standard sizes in non-stock colors or grilles take two to four weeks. Custom sizes run six to ten weeks, and winter orders slow by another two to three weeks industry-wide. Plan your opening to one of the typical window sizes when possible — your project schedule and your budget will both thank you.
Verify screens, grilles, and hardware finishes at order time; those trim items often ship separately and can delay a certificate of occupancy if missing at the final inspection.
