Jorick said
Find a circle that you know the circumference and diameter of. Find their ratio. Tah-dah, you've calculated pi.
Thats awful. That requires you to get string, a ruler, and a compass to measure a circle, and your result will only be as accurate as your ruler at the end, because you run into the same problem again.
Slightly better is to use the infinite sum [4/1 - 4/3 + 4/5 - 4/7 + 4/9 +... 4/2n-1] whose value converges on pi as n approaches infinity.
Or any other method you could find with google.