N = N* fp ne fl fi fc fL

Where N represents the number of stars in the Milky Way Galaxy -

fp is the fraction of stars that have planets around them;

ne is the number of planets per star that are capable of sustaining life;

fl is the fraction of planets in ne where life evolves;

fi is the fraction of fl where intelligent life evolves;

fc is the fraction of fi that communicate;

fL is fraction of the planet's life during which the communicating civilizations live;

to arrive at N - the number of communicating civilizations in the galaxy.