N = The number of civilizations in The Milky Way Galaxy whose electromagnetic emissions are detectable.
R * =The rate of formation of stars suitable for the development of intelligent life.
f p = The fraction of those stars with planetary systems.
n e = The number of planets, per solar system, with an environment suitable for life.
f l = The fraction of suitable planets on which life actually appears.
f i = The fraction of life bearing planets on which intelligent life emerges.
f c = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
L = The length of time such civilizations release detectable signals into space.