Drake Equation
A probabilistic equation formulated by Frank Drake in 1961 to estimate the number N of active, communicative extraterrestrial civilizations in the Milky Way.
Governing formula
N = R* · f_p · n_e · f_l · f_i · f_c · L
SI unit
Number of Civilizations N
In depth
Multiplies factors: star formation rate R*, fraction with planets f_p, habitable planets n_e, life emergence f_l, intelligent life f_i, technological communication f_c, and civilization lifetime L.
Examples in the real world
Estimating active communicating technological civilizations in our galaxy ranging from 1 to thousands depending on assumed lifetime L.