Drake's Equation

N = R* x fp x ne x fl x fi x fc x L

R* = (number of new stars per year in the galaxy, on average)

fp = (fraction of those stars with planets)

ne = (number of life-capable planets per planetary system)

fl = (fraction of life-capable planets that develop life)

fi = (fraction of life that evolves into intelligent life)

fc = (fraction of intelligent life that invents radio communication)

L = (lifespan of a radio-possessing civilization, in years)