# Get $\pi$ decimals manually

I know that there was a guy that could get 100 decimal digits of $\pi$ before computers were able to get thousands.

How did the guy do that?

• At least two ways: Machin's formulae or the arithmetic-geometric mean. See this. – J. M. is a poor mathematician Dec 1 '10 at 12:45
• Sorry.. duplicated :( – Diego Dec 1 '10 at 12:47

According to Wikipedia, John Machin combined the formula $$\frac{\pi}{4}=4\cot^{-1}5-\cot^{-1}239$$ with the Taylor series expansion for the inverse tangent in order to compute $\pi$ to 100 decimal places.
This page on the chronology of pi contains many useful notes on how the pre-computer era calculations of $\pi$ were performed.