We can estimate the number of civilizations existing based on some basic facts. The first variable needed in this estimate is the number of planets in our galaxy on which life can sustain; this variable would be called NEP, meaning the number of Earth-like planets. Then, we need a number of such plants that can actually have life; this variable would be called flife, indicating a fraction of planets having any lifeform. Multiplying these two would tell us the number of planets having life. Then we have a variable that tells us the number of planets where life evolves and forms a civilization; this variable would be called fciv, a fraction of planets forming civilization.