Upapada Lagna (UPL) is a derived ascendant used in classical Vedic astrology primarily for assessing marriage timing, spouse characteristics, and certain life events (e.g., consummation, childbirth). This paper explains the conceptual basis, traditional sources, mathematical derivation, step-by-step computational algorithm, worked examples, caveats, and implementation notes suitable for building an automated Upapada Lagna calculator.