When taking measurements use the rough opening.  If the rough opening is not accessible: Exterior door, measure the door and add 1-1/2 inches for the width and for the height add 1 inch for an outswing and 2 inches for an inswing

Interior doors: Measure the door add 1-1/2inches for the width and add 2 inches for the height example: 30" door would be 31-1/2 x 82