Drake equation

This is the equation Frank Drake wrote on the blackboard in 1961:

N = R f_p n_e f_l f_i f_c L

Each term to the right of the equals sign represents one of the questions we asked earlier:
R is the rate of star formation within the Galaxy, expressed in stars per year (approx.: 100 billion);
f_p is the fraction of stars that form planets (approx.: 20% to 50%);
n_e is the average number of planets each such star possesses, which are capable of supporting life (from 1 to 5);
f_l is the fraction of those planets where life actually occurs (from 100% -resilience of life- to 0%);
f_i is the fraction of life-bearing planets where intelligence arises (from 100% to 0%, depending on your assumptions);
f_c is the fraction of intelligent life-bearing planets where intelligent beings develop the ability to communicate beyond their own world; and
L is the length of time, in years, that such communications remain detectable.

By estimating values for each of these terms and multiplying them together, one arrives at:

N, the estimated number of detectable civilizations.

TO CALCULATE 'N' follow this link: Drake equation calculator