2 inches wider than what you want to put in the closet.

I normally dimension reach-in clothes closets at 2'-1" face of stud to face of stud for hinged doors, deeper for pocket or bi-pass doors.

IMO the interior is the space you want to have for hangers and that is min 24"

When I look up framing instructions, they give closet depth based on the measurement from the finished face of the back wall to the outer dimension of the outer wall studs, not the finished face.

The framing plan goes from one side of one wall to the same side of the next wall, which is what you described above. The distance between those walls in the framing plan should be larger than the designed closet depth. So if these are 2x4 walls the closet width will be 4.5" less than the width on the framing plan.

@bry911 Thanks. Here's some pictures of what I meant. I guess what's tripping me up is that it's measured from a drywall face to a stud face.

What is shown in the first picture is a 26" deep closet. That is not really a framing plan though, it is just a diagram and is fine to follow if you want a 26" deep closet.