Nominal sizes for doors are typically based on standard measurements and are used as a reference point for builders, contractors, and other professionals in the construction industry.

In the US, nominal sizes for doors are typically based on the measurement of the door's width in inches, and are usually rounded to the nearest whole number. For example, a nominal size 32" door is meant to fit into a rough opening that is approximately 32 inches wide. However, the actual width of the door may vary slightly from the nominal size, as the size is only a reference point and is not meant to be an exact measurement.

It is important to note that nominal sizes for doors are not always the same as the actual dimensions of the door. In some cases, the nominal size may be slightly larger or smaller than the actual dimensions of the door. It is always important to measure the rough opening carefully and use the actual dimensions of the door to ensure a proper fit. 

For example, a 2’-8” wide door can be

• 32”
• 31-¼”
• 31- ¾”

This is the actual door size & does not include the frame thickness. For Example, the above 32” wide door can be 33-½” (net frame size) once it is hung on a frame.  

In this case, the nominal size for the door unit is 47" x 80" 

the nominal size of 47" x 80" refers to the combined width of a 32"x80" and a 15"x80" sidelite , and does not refer to the rough opening size. The Net Frame Size , which is 48-1/2" x 81-3/4", is the size of the unit after it has been fully assembled with a frame.