The marks for the final ranking in the UPSC Civil Services Examination are calculated based on three main components: the Preliminary Examination, the Mains Examination, and the Interview (Personality Test). The preliminary exam is qualifying, while the Mains carries significant weight, with each paper contributing to the final score. Finally, the Interview assesses candidates' personality and aptitude for civil services, further impacting their overall ranking. A cumulative score from these stages determines the selection list.