Did you know ? Ludolf's number, colloquially (pi) or, exceptionally, Archimedes' constant is the ratio of the circumference of a circle to its diameter. Whether the circle is small or large, ( pi ) is still the same, it is a mathematical constant. This constant is commonly used not only in mathematics but also in physics, engineering and other sciences. Ludolf's number is an irrational, transcendent number whose first three digits are 3.14.