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How does the Drake equation estimate the number of communicating civilizations in the Milky Way Galaxy?

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artist's conception of Kepler-186f — extraterrestrial intelligence An artist's conception depicts Kepler-186f, the first Earth-sized extrasolar planet found in its star's habitable zone.

The Drake equation is a mathematical formula used to estimate the number (N) of active, technically advanced civilizations in the Milky Way Galaxy capable of interstellar radio communication. Formulated by astrophysicist Frank Drake in 1961, the equation calculates N as the product of seven factors:123

$R_{*}$ : The average rate of star formation in the galaxy.12
$f_{p}$ : The fraction of those stars that possess planetary systems.12
$n_{e}$ : The average number of planets per system ecologically suitable for life.12
$f_{l}$ : The fraction of suitable planets where life actually develops.12
$f_{i}$ : The fraction of life-bearing planets where intelligent life evolves.12
$f_{c}$ : The fraction of intelligent civilizations that develop technology to transmit detectable signals into space.12
$L$ : The average lifetime of such a technical civilization.12

Because many of these variables remain highly uncertain, the equation serves more as a framework for evaluating the possibilities of extraterrestrial intelligence rather than providing a precise count. Estimates for N vary wildly depending on the assumed value of L, ranging from a solitary civilization (our own) to millions.21