Formula to calculate how many civilizations there might be in our galaxy that we could communicate with.

Low estimate numbers in the formula gives 20 civilizations. High estimates give 50 000 000.

The [[rare earth hypothesis]] adds a lot more factors, like moons for tides, gas giants for protection, absence of radiation, plate tectonics among them

The formula's terms:^{1}

**N**\= The number of civilizations in the Milky Way galaxy whose electromagnetic emissions are detectable.

**R***\= The rate of formation of stars suitable for the development of intelligent life (number per year).

**f p**\= The fraction of those stars with planetary systems.

**n e\=** The number of planets, per solar system, with an environment suitable for life.

**f l**\= The fraction of suitable planets on which life actually appears.

**f i**\= The fraction of life bearing planets on which intelligent life emerges.

**f c**\= The fraction of civilizations that develop a technology that produces detectable signs of their existence.

**L**\= The average length of time such civilizations produce such signs (years).