Why wood and vinyl window sizes differ

Riddle: When is three feet not three feet? Answer: When it's the width of a window.

If you are shopping for windows, you may have noticed that standard stock sizes vary by material. A three-foot-wide window made of wood, for example, is a different size than a three-foot-wide window made of vinyl or aluminum -- and neither measures 36 inches!

Windows are sized in feet and inches. The "call size" is a number used to represent both feet and inches of the height and width of the window. A call size of 3050, for example, is a 3 ft. 0 in.-by-5-ft. 0 in. window. But that's an approximate size. Windows don't usually match their call sizes precisely. Wood windows are larger, and vinyl windows are smaller. Why is that?

A brief history of window sizes

Window sizes are a leftover convention from before WWII. Wood windows were originally made on site, and later they were made locally by a neighborhood lumberyard, millwork distributor or manufacturer. Because glass is the primary component of windows and wood was the frame and sash material of choice before the advent of aluminum, a window was the size of its visible glass.

If the glass size was three-feet wide, the window was also three-feet wide; regardless that the whole window, including the sash and frame, was several inches wider. In our 3050 example, just about three feet of glass would be visible -- less the small slice of glass attached inside the wood of the sash. In a wood window, the sash frame and window frame add about 1½ inches, making a so-called "three-foot" wood window actually around 37½ inches wide instead of 36 inches.

Aluminum and vinyl window sizes

During World War II, American manufacturers learned how to efficiently extrude aluminum for airplane parts. When the war was over, rather than close the factories, aluminum parts manufacturers began making storm windows and doors, which are measured and made to exact sizes. It wasn't long before they were making regular aluminum windows. Like storm windows, aluminum windows were designed to match the size requested; i.e., a three-foot window was actually 36 inches (less ½ an inch for the rough opening.)

Aluminum window manufacturers began making vinyl windows when aluminum became too costly. It was only natural that they would adopt the same sizing convention as they used for aluminum windows. Vinyl and aluminum windows are closer to their call sizes than wood windows: a 3,050 vinyl window is 35½ inches-by-60 inches.

Still confused? Just remember that vinyl or aluminum window measurements include the entire unit -- glass, frame and sash -- while wood window sizes go by the measure of the glass. You'll have more visible glass in a three-foot wood window than a three-foot vinyl window, but you'll also have a larger window unit in wood.