The number Pi (3.14159...) is derived from the ratio of a circle's circumference to its diameter. Historically, Archimedes calculated Pi using circumscribed polygons, achieving three-digit accuracy with a 96-sided polygon. Modern calculations of Pi utilize infinite series, a method first formulated in the mid-1600s by Newton. The discussion highlights the evolution of Pi's calculation methods over time. Understanding Pi's derivation showcases both historical and contemporary mathematical techniques.