How many decimal places does pi go out?

Answer 1

Infinitely many. This is because pi is an irrational number, which means the decimal places neither terminate (such as 2.5) nor repeat (such as 0.333...). This also means that pi cannot be expressed as the fraction of two integers (such as 4/9). The proof of this dates back to the 18th century.

At the moment, pi has been computed to more than a trillion (1 000 000 000 000) digits.

For most practical purposes, just the first few digits are enough. Rounding pi to 3.14 gives an error of

pi - 3.14 = 0.00159265...

which is suitable for common use. Pi truncated to 11 digits could measure the circumference of the earth with the precision of a millimeter.