where:

N is the number of civilizations in our galaxy with which we might expect to be able to communicate at any given time

and

R* is the rate of star formation in our galaxy

f_p is the fraction of those stars that have planets

n_e is average number of planets that can potentially support life per star that has planets

f_l is the fraction of the above that actually go on to develop life

f_i is the fraction of the above that actually go on to develop intelligent life

f_c is the fraction of the above that are willing and able to communicate

L is the expected lifetime of such a civilization