Not necessarily. Generally will have trim at the edge of the door, so if you take the trim off you will get the actual size of the rough opening. If you went to Home Depot and were looking for a interior door, and your rough opening (stud to stud width and floor to header length) is 31”x81”, you would buy a 30”x80” door, once installed you would trim 2 sides and top with casing. The extra inch on long measurement usually requires some slope in case on an entrance/exterior door to protect from water infiltration and flooring. On an interior door, the extra inch accounts for flooring as well. Now if for some strange reason there is no trim, in your photo it has trim, measuring the door probably would work. If you measure the door that should give you a close enough idea on size. First step should be removing the trim. The reason for the door being smaller than the opening is so that you can shim, insulate and weather strip. You don’t want the door right up against the Jack or King Studs, it will bow the jamb when you screw to framing and mess with how the door sits in the frame (jamb) and affect operation.