Frank Drake devised the "Drake equation" - a formula to calculate how many civilisations might be broadcasting radio signals. This is the Drake equation.

N = R_* x f_p x n_e x f_l x f_i x f_c x L

R_* is the rate starbirth.
The number of new stars formed in the Galaxy. This is widely accepted by astronomers to be 10 per year.
f_p is stars with planets.
The percentage of those new stars that form planets.
n_e is habitable planets.
The number of those planets that are the right distance from their star, and of the right size to be suitable for life.
f_l is planets with life.
The percentage of those planets on which life has actually evolved.
f_i is intelligent life.
Percentage of planets where life forms have evolved sufficiently for "intelligent" creature to exist.
f_c is communicable life.
The percentage of those planets on which intelligent lifeforms have developed technologically so they can communicate with other civilisations.
L is life span of civilisation.
The length of time that a technologically advanced civilisation exists and in which it can send a potentially receive signals.