Who Discovered Pi?
Nobody, exactly — and that's the honest answer. Pi wasn't invented by one person on one day. The ratio was staring at every civilization that ever rolled a wheel. What changed over the centuries wasn't the discovery of pi, but its precision and its notation.
Pi was found, not invented
Here's the key idea up front: pi is a fact about circles, baked into the geometry of the universe. Any culture that measured a round object noticed that its circumference was always a little more than three times its width. So pi wasn't "discovered" the way a new continent is — it was independently bumped into by many peoples, again and again, long before anyone had a name or a symbol for it. The story of pi is really the story of people pinning it down ever more tightly.
Babylonians and Egyptians (~1900–1650 BCE)
The oldest surviving estimates go back nearly four thousand years. The Babylonians used a value of about 3.125 (they wrote it as 3 + 1/8). Over in Egypt, the Rhind Mathematical Papyrus — a scribe's copy from around 1650 BCE — implies a value of about 3.16, arrived at by comparing a circle to a square. Neither was exact, but both were solid working numbers for building and surveying, and both prove that pi was in use well before the Greeks.
Archimedes (~250 BCE)
If anyone deserves credit for first "calculating" pi rigorously, it's Archimedes of Syracuse. Instead of measuring a circle, he trapped it between polygons drawn inside and outside it, then added sides to tighten the bounds. With a 96-sided polygon he proved pi lies between 223/71 and 22/7 — correct to two decimals, and, crucially, backed by airtight logic rather than a tape measure. This is why pi is sometimes called Archimedes' constant. He didn't discover that circles have a ratio; he was the first to prove how big it is.
Zu Chongzhi (~480 CE)
Working in China a few centuries later, the astronomer Zu Chongzhi pushed pi to seven correct digits — 3.1415926 to 3.1415927 — a level of precision no one in the world would beat for nearly a thousand years. He also gave the elegant approximation 355/113, accurate to six decimal places, which remains one of the best simple fractions for pi ever found.
Madhava and the Kerala school (~1400)
In southern India, the mathematician Madhava of Sangamagrama and his followers made the next conceptual leap: they expressed pi as an infinite series, an endless sum that inches toward the true value. This is the same idea Europeans would rediscover centuries later as the Leibniz series. Madhava had, in effect, invented a machine for grinding out pi to any precision you had the patience for.
The symbol π (1706)
For all these millennia, nobody wrote "π." The number was described in words. That changed in 1706, when the Welsh mathematician William Jones first used the Greek letter π as shorthand for the ratio. It caught on largely because the great Leonhard Euler adopted it a few decades later — and when Euler used a symbol, the mathematical world followed. So the symbol for pi has an inventor and a birthday, even though the number itself does not.
Proving what pi is (1761 and 1882)
The final chapters are about understanding pi's nature. In 1761, Johann Lambert proved pi is irrational — it can't be written as a fraction, and its digits never repeat. Then in 1882, Ferdinand von Lindemann proved pi is transcendental, settling the ancient puzzle of "squaring the circle" once and for all (you can't). These weren't discoveries of pi so much as discoveries about it.
The computer era
From the twentieth century on, the chase went electronic. Early computers cracked thousands of digits; modern software like y-cruncher has reached past 100 trillion. For the how, see how pi is calculated.
So, who invented pi?
No one — and "invented" is the wrong word. Pi is discovered and inherent, part of the fabric of geometry itself. What humans developed over four thousand years were the tools to pin it down: sharper methods, better approximations, the symbol π, and proofs of what kind of number it is. Pi was always there. We just kept getting to know it better.