N* = the number of stars in the Milky Way galaxy: 1 billion 10 billion 100 billion 200 billion 300 billion 400 billion 500 billion 600 billion

fp = fraction of stars with planets around them: .0001% (1 of 1,000,000) .001% (1 of 100,000) .01% (1 of 10,000) .05% (1 of 2,000) .1% (1 of 1,000) .5% 1% 2% 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

ne = number of planets per star ecologically able to sustain life: .33 .5 .67 1 1.5 2 3 4

fl = fraction of those planets where life actually evolves: .0001% (1 of 1,000,000) .001% (1 of 100,000) .01% (1 of 10,000) .05% (1 of 2,000) .1% (1 of 1,000) .5% 1% 2% 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

fi = the fraction of fl that evolves intelligent life: .0001% (1 of 1,000,000) .001% (1 of 100,000) .01% (1 of 10,000) .05% (1 of 2,000) .1% (1 of 1,000) .5% 1% 2% 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

fc = the fraction of fi that communicates: .0001% (1 of 1,000,000) .001% (1 of 100,000) .01% (1 of 10,000) .05% (1 of 2,000) .1% (1 of 1,000) .5% 1% 2% 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

fL = the fraction of the planet's life during which the communicating civilizations survives: 1 / 1,000,000,000th (10 years) 1 / 100,000,000th (100 years) 1 / 10,000,000th (1,000 years) 1 / 1,000,000th (10,000 years) 1 / 100,000th (100,000 years) 1 / 10,000th (1,000,000 years) 1 / 1,000th (10,000,000 years) 1% (100 million years) 10% (1 billion years) 20% (2 billion years) 30% (3 billion years) 40% (4 billion years) 50% (5 billion years) 100%