UPSC ranks candidates in the civil services exam based on their performance across three stages: Preliminary, Mains, and Interview. The preliminary stage serves as a qualifying exam, while the Mains consists of numerous written papers that assess the candidates' knowledge and analytical abilities. Each candidate receives a score based on their performance, which contributes to a merit list. The final rank is determined by combining the scores from the Mains and Interview stages, reflecting a candidate's overall capability and suitability for civil services. This systematic ranking ensures a fair selection process based on merit.