Picking the right windows size is harder than it sounds because window manufacturers list three different dimensions for the same product — nominal size, rough opening, and actual unit size — and they vary by brand. A 3030 window from Andersen is not the same size as a 3030 from Pella or Marvin. This guide covers the standard sizing systems, egress and code requirements, how to measure for replacement, and the realities of ordering custom sizes when your openings don’t match a catalog.
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How Window Sizes Are Labeled
Most window manufacturers use a four-digit nominal size code. The first two digits indicate width in feet and inches, the last two indicate height. A 2840 window is 2 feet 8 inches wide by 4 feet 0 inches tall. A 3050 is 3 feet 0 inches by 5 feet 0 inches. The nominal dimensions describe the rough opening size, not the actual window unit.
The rough opening is typically 1/2 inch larger in both directions than the unit size to allow for shimming, insulation, and squaring. So a 3050 window has a rough opening of 36 by 60 inches and a unit size of roughly 35-1/2 by 59-1/2 inches. Always verify with the manufacturer’s spec sheet because tolerances vary between Andersen, Pella, Marvin, JELD-WEN, and Milgard.
Standard Sizes by Window Type
Double-hung windows, the most common residential style, come in standard sizes from 24×36 inches up to 48×72 inches in 2-inch increments. Sliders range from 36×24 to 96×60. Casements run smaller, typically 18×24 up to 36×72. Picture windows go larger, from 36×24 up to 96×60 or wider for custom.
Bay windows assemble from three or four flanker units mounted at 30 or 45 degree angles, producing total widths of 78 to 144 inches and projecting 12 to 24 inches from the wall. Bow windows use four or five flat units in a curved arrangement and produce total widths of 96 to 168 inches. Both bay and bow units typically require structural headers and exterior brackets for proper support.
Egress Code: The Sizing Requirement That Matters Most
IRC R310.2 requires every sleeping room to have at least one egress window meeting four criteria: minimum 5.7 square feet of clear openable area (5.0 in basement and ground-floor windows), minimum 24 inches of clear height, minimum 20 inches of clear width, and a sill height no more than 44 inches above the floor.
The trap is that “clear opening” means the unobstructed area when the window is fully open — not the rough opening or unit size. A double-hung window must be sized so that when the lower sash is fully raised, the open area exceeds 5.7 square feet. That typically means a unit size of 32×54 inches minimum for double-hung egress, or 28×48 for casement (which provides more clear area per unit size because the entire sash swings out).
Measuring for Replacement Windows
For full-frame replacement, measure the existing rough opening width at the top, middle, and bottom; use the smallest dimension. Then measure the height at left, center, and right; again use the smallest dimension. Subtract 1/2 inch from each to get the replacement window unit size you should order.
For insert (pocket) replacement, measure the existing frame opening — the visible interior frame, not the rough opening. Measure width at top, middle, and bottom; height at left, center, right. Use the smallest dimensions and subtract 1/4 inch for a snug fit. Insert replacement keeps the existing exterior trim and interior frame but reduces the visible glass area by roughly 2 to 3 inches in each direction.
Brand Spotlight: Andersen, Pella, and Marvin
Andersen 400 Series is the volume leader in mid-priced residential windows. Standard double-hung sizes start at $400 to $700 per window installed, with a Fibrex composite frame backed by a 20-year limited warranty. Their A-Series in fiberglass costs $700 to $1,200 per opening and competes at the premium end.
Pella Lifestyle Series wood-clad double-hung runs $500 to $900 installed and offers integrated blinds between glass, which simplifies window treatment maintenance. Marvin Elevate wood-clad double-hung sits at $800 to $1,400 installed and uses Ultrex fiberglass on the exterior with wood interior. Marvin Signature Ultimate, the all-wood premium line, runs $1,200 to $2,400 per window and is the choice for high-end historical restoration where authentic wood profiles matter.
Energy Performance and Glass Options
The Energy Star certification varies by climate zone. In the Northern zone (most of the US north of Tennessee), look for U-factor of 0.27 or lower and SHGC of 0.40 or higher. In the Southern zone, U-factor of 0.40 or lower and SHGC of 0.25 or lower. The numbers appear on the NFRC label affixed to every new window.
Double-pane low-E argon-filled glass is the current standard. Triple-pane adds another $80 to $200 per window and reduces U-factor to roughly 0.18 to 0.22, which pays back over 10 to 15 years in cold climates. Laminated glass, often used on first-floor windows or near doors for security, adds roughly $50 to $120 per window and meets impact-resistance codes in hurricane zones.
Custom Sizing and Lead Times
Standard sizes ship from regional warehouses in two to four weeks. Custom sizes — anything not in the catalog — typically require 6 to 12 weeks from the manufacturer. Custom width or height beyond catalog sizes adds 15 to 40 percent to the unit price.
For old houses with unusual openings, the smart move is often to frame down to a standard size rather than order custom. Adding 2 inches of trim around a 30×52 inch opening to fit a stock 28×48 inch window costs $40 in lumber and an hour of labor, versus $200 to $500 in custom upcharge. Reserve custom orders for truly unique openings — round-top, arch, or trapezoid windows where stock options don’t exist. Get the exact rough opening dimensions to the installer before ordering anything, because window manufacturers will not warrant a unit that doesn’t match the order spec.
