Home improvement and some window retailers carry windows identified by whole numbers for standard-sized rough openings. Though the actual window measurements are usually 1/2 inch less, the identifier is always a whole number for the rough opening. For example, sliding windows designed to fit into a 2-by-2-foot rough opening are identified by the number 2020, but have actual measurements of 23 1/2 by 23 1/2 inches. Windows that fit rough openings of 44 by 44 inches are identified as "3838" windows, because they are actually 3 feet, 8 inches wide by 3 feet, 8 inches long -- not including the 1/2 inch taken off for the actual window size. The "3838" also identifies the window schedule on the blueprints.
A Half-Inch Less
A manufacturer identifies its windows by rough opening sizes but takes off a 1/2 inch on both window dimensions to ensure ease of installation. When you install a window, the rough opening must be fitted with flashing materials, and then the window must be centered in the space to ensure it functions properly. The 1/2-inch reduction in the size of the window on both dimensions allows the installer the room to do just that. Once the window is plumb, installers add shims to it to keep it in place while they secure the window to the rough opening. Each manufacturer may offer different sizes, but most provide windows with standard measurements.
While you can definitely order windows made to custom sizes, when you build a home or add new windows, you can buy them "off the shelf" if you have standard-sized rough openings. For example, all windows are first fit by the horizontal width, then their height. Standard sliding window widths are 36, 48, 60, 72 and 84 inches wide; the windows themselves are actually 36 1/2, 47 1/2, 59 1/2, 71 1/2 and 83 1/2 inches wide to allow for installation. Standard heights are 24 inches, 36 inches, 48 and 60 inches. A 6-foot-wide by 4-foot-tall window is identified as "6040."
Picture windows have similar standard dimensions, but with more options from which to choose. These windows don't open, so they can be much wider and taller. Standard widths are 2 feet, 3 feet, 4 feet, 5 feet, 6 feet and 8 feet. Standard heights in whole numbers range from 2 feet tall all the way 8 feet tall, with additional window heights of 18 inches, 52 inches, 54 inches and 62 inches. Again, rough openings are in whole numbers, with 1/2 inch taken from each rough opening size for the actual window size.
Rough opening widths for double-hung windows start at 24 inches, ranging all the way up to 48 inches, with additional window widths of 28 inches, 32 inches and 40 inches. Again, the actual window size is 1/2 inch less than its rough opening. Double hung window heights vary from 36 to 72 inches tall, including window heights of 44, 52, 54 and 62 inches with 1/2 inch taken off each measurement for the actual window size.