Average double-hung windows range from 24 to 48 inches, while average heights range from 36 to 72 inches. Window suppliers and home centers generally carry windows that give the dimensions of the window's rough opening. The windows actual size is typically half an inch smaller in both dimensions.
A window opening size is given in feet and inches, and rounded up to the nearest inch. For instance, a window that fits into a 2x3-foot rough opening would be called a 2030, because the opening is two feet and zero inches by three feet and zero inches. Likewise, a window that fits into a 4.5x3.5-foot rough opening would be called a 4636, because it is four feet and six inches by three feet and six inches.