Zu Chongzhi

A Chinese mathematician and astronomer who, around 480 CE, computed $\pi$ to seven decimal places ($3.1415926$) using the same inscribed-and-circumscribed-polygon method that Archimedes had used seven centuries earlier (with $96$-sided polygons; Zu went much further, to $24,576$-gons). His seven-decimal value stood as the world record for almost a thousand years. He also gave an excellent rational approximation, $355/113$, that matches $\pi$ to six decimal places: handy when fractions were the only practical written form for a precise number.