how do you know that pi is an irrational number..How many decimal points has anybody calculated it to.?

Not only is pi irrational, it is transcendental. For example,the square root of 2 is irrational, but sqrt(2) satisfies the equation x^2-2=0. It is known that pi does not satisfy any polynomial equation with integer coefficients.

The current limit for computed digits of pi is over 1 trillion digits:

http://mathworld.wolfram.com/PiDigits.html

They actually test computers, supercomputers, on how many places they can compute pi to. Don't know what the record is now, but it's not infinite.

Somebody called Lambert proved this back in about 1760, so it can't have been all that difficult. No mathematician has disagreed with him since, so I would suppose he was right.

because the number cannot be put into a fraction format. last time i looked it's in the billions of numbers.

It has been asked and answered several times in Yahoo Answers.