N* = the number of stars in the Milky Way galaxy:

fp = fraction of stars with planets around them:

ne = number of planets per star ecologically able to sustain life:

fl = fraction of those planets where life actually evolves:

fi = the fraction of fl that evolves intelligent life:

fc = the fraction of fi that communicates:

fL = the fraction of the planet's life during which the communicating civilizations survives:

N = the number of communicating civilizations in the galaxy =