In mathematics, the ratio of the circumference of a circle to its diameter. An irrational number (see alsotranscendental number), it has an approximate value of 3.14, but its exact value must be represented by a symbol, the Greek letter . Pi is used in calculations involving lengths, areas, and volumes of circles, spheres, cylinders, and cones. It also arises frequently in problems dealing with certain periodic phenomena (e.g., motion of pendulums, alternating electric currents). By the end of the 20th century, computers had calculated pi to more than 200 billion decimal places.