Drake Equation:

N = R* fp ne fl fi fc L

where,

* N = The number of communicative civilizations
* R* = The rate of formation of suitable stars (stars such as our Sun)
* fp = The fraction of those stars with planets. (Current evidence indicates that planetary systems may be common for stars like the Sun.)
* ne = The number of Earth-like worlds per planetary system
* fl = The fraction of those Earth-like planets where life actually develops
* fi = The fraction of life sites where intelligence develops
* fc = The fraction of communicative planets (those on which electromagnetic communications technology develops)
* L = The 'lifetime' of communicating civilizations

Calculation of Drake Equation is made easier. Free Online Science Calculators.