Archimedes (287 - 212 B.C.)

In his Measurement of the Circle Archimedes found an approximation value for pi by approximating the circumference of the unit circle by the perimeter of inscribed and cirumscribed polygons. Using polygons with 96 sides he found
    3.1408 < pi < 3.1428
This was just one of Archimedes' many accomplishments. Among others are Archimedes anticipated by 1800 years many developments in the calculus.