Understanding door sizing:

Nominal size is the commonly referred to size of the door slab within the industry. This number is the closest even number to the actual size, similar to how a 2x4 is actually 1.5”x3.5”

Actual size refers to the actual size of the door slab without the frame.

Assembled size refers to the actual size of the entire prehung door with the frame included. This is measured from the outside of frame also called “tip to tip”

Measuring your door:

Find your door size: To properly measure your door, open the door and measure the width and height of the door slab, do not include the frame in this measurement. This will give you an actual size, round to the nearest whole number to get your standard/nominal size.

Find your jamb size: The only part of the frame that needs to be measured is the width, this it usually 4-9/16” or 6-9/16”. Be sure to not include any trim in this measurement. (see images for jamb width)