Originally Posted by Big Dave
I would take each door down one at a time and set a new door next to it and trace the placement of hinges on the new door. Make sure to align the top and notice if there is any difference in length. I would probably buy the doors without any door knob hole so you can drill and place it at the exact same place as your existing strike. That way too you don't have to worry about left or right swing.
I prefer to set the new blank in the existing jamb to mark the hinge locations. While keeping the hinge side tight aginst the jamb, you can slide the door up to the head jamb with a pry bar under your foot or a couple of wedges, and see if it fits true, if not, you can trim the top of the door parallel, then reinsert to mark the hinges.Don't forget to insert a spacer at the head before marking the hinges. The black plastic Porter Cable jig works good, for about $20. Make a couple of practice cuts on the old door edge to get comfortable with locating the jig, and take your time, they will rock easily if you are not carefull with the router.