Standard Window Sizes: How to Read Size Codes

There is no single national list of standard window sizes. Each manufacturer publishes its own chart, and most name a size with a four-digit code: the first two digits are the width in feet and inches, the last two the height. Pella's example is a 2438, which is 2 feet 4 inches wide by 3 feet 8 inches tall, or 28 by 44 inches ¹. Common double-hung codes on Pella's list include 2030, 2044, 2840 and 2852 ¹.

The catch is what the code measures. Depending on the maker, the code may describe the rough opening (the framed hole in the wall) or the window unit itself, and the two are not the same size ^{1 2}. Before you order, or compare quotes, check which one a size refers to.

How to read a window size code

A code such as 3046 reads as 3 feet 0 inches wide by 4 feet 6 inches tall. Fixr gives the example of a 2044: 2 feet wide by 4 feet 4 inches tall ⁴. Width always comes first.

Not every chart uses feet and inches. Viwinco's charts state that its callout sizes are expressed in inches and equal the rough opening, so its numbers are read directly as inches of opening ². Ply Gem's double-hung guide uses product codes such as DH2040 and lists rough opening, unit dimension and clear opening in separate tables ³. Two windows with similar-looking codes from two makers may not be the same size, which is why a manufacturer's chart is the place to confirm a number.

Rough opening vs. unit size

The rough opening is the framed opening the window sits in. The unit size is the outside of the window frame. The opening is a little larger than the unit, and insulation fills the gap between them ⁴.

How much smaller depends on the product. In Viwinco's single-hung chart, a 20-inch-wide rough opening takes a 19½-inch unit, a 36-inch opening a 35½-inch unit, and so on across the chart: the unit is half an inch narrower than the opening ². Ply Gem describes its modular units as built to traditional wood window rough opening dimensions ³. Other lines use other allowances, so the chart for the exact series is what matters.

There is a third measurement on many charts: the clear or net opening, the space you could pass through with the sash open. That is the number used for bedroom escape (egress) openings, and Ply Gem's guide marks the sizes that meet the model code's egress dimensions while telling readers to confirm with the local code official ³. Our egress window cost article links the code text itself.

Common sizes by window type

The table below lists the example sizes in Pella's guide and in Fixr's article, by type ^{1 4}. They are common sizes, not the only ones.

Bay and bow windows have no fixed size list, because they are built from several units joined together. Pella describes a typical bay as a center window about half the total width with a side window of about a quarter on each end, set at 30 or 45 degrees ¹. More on how each type opens is in types of windows, and on the joined units in bay vs. bow windows.

Standard, stock and custom

"Standard" does not mean "on a shelf." Pella notes that standard-size windows are still made to order after you buy them; standard simply means a size that fits most new homes ¹. A stock window is one a store keeps on hand, usually in a short list of sizes.

A custom window is built to the opening you have. In a replacement, the existing opening is often not a current standard size, especially in older houses. Pella says a custom window may cost less in that case than changing the wall to fit a standard size ¹. Fixr says a custom window costs more than a standard one but does not give a figure, and it reports that a carpenter may charge $58 to $106 per hour to resize an opening ⁴. Special shapes, such as arches and circles, cost more because the frame and glass are built individually ¹.

So the choice is usually between two costs: a made-to-size unit, or a standard unit plus carpentry, trim and siding work to change the opening. Ask each installer which they are quoting.

How to measure an existing window

For an estimate, Fixr suggests this sequence ⁴:

1. Check the frame is square by measuring both diagonals; they should match within 1/16 to 1/8 inch.
2. Measure the width between the jambs at the top, middle and bottom, and keep the smallest.
3. Measure the height at the left, middle and right, and keep the smallest.
4. Round down to the nearest 1/8 inch.

Fixr notes that depth matters mainly for mobile or modular homes ⁴; our mobile home window replacement article covers those. Your own numbers are good enough to request quotes, but Fixr says final measurements are taken by a measuring technician to make sure the window fits ⁴, and Pella warns that a small error on an uncommon size can be expensive ¹. Whether the new window is an insert inside the old frame or a full-frame unit in the rough opening changes which size is ordered; see full-frame vs. insert window replacement.

Before you get quotes

List each window by room with its type and your measured width and height, and note whether it is a bedroom window that has to work as an exit. Ask each installer whether the price is for a standard or a custom size, and whether any wall or trim work is included. How to compare window quotes covers what else a written quote should show, and window replacement cost covers whole-house prices. When you are ready, you can compare quotes from window installers.

We are not affiliated with the manufacturers named.

About these figures

• ¹ Reported: Pella's size-code explanation (2438 example), example sizes by type, bay proportions, and notes on standard, in-stock and custom windows, updated May 21, 2026, checked October 26, 2026.
• ² Reported: Viwinco's 2026 S-Series size charts, callout sizes equal to rough opening and unit sizes half an inch smaller, checked October 26, 2026.
• ³ Reported: Ply Gem's double-hung modular sizing supplement (rough opening, unit dimension and clear opening tables, egress markings), checked October 26, 2026.
• ⁴ Reported: Fixr's size notation example, example sizes by type, measuring steps and the $58 to $106 hourly carpenter rate for resizing an opening, published June 24, 2024, checked October 26, 2026.