I am using
calc often (from the terminal) and I was wondering if there is a PI constant in there, predefined, somewhere?
you also can choose kcalc there is a scientific - mode in kcalc there again you can choose mathematical constants e.g. PI, Euler, golden ratio ...
if you want to tickle more out of PI in own written program you can use then the library of http://gmplib.org/ this library you can use when compiling with gcc resp. g++
have fun !