R - Rate of formation of suitable stars in our galaxy (aggregate number)

R =

1 billion 10 billion 100 billion 200billion 300 billion 400 billion 500 billion 600 billion

f_p - Percentage of those stars with planets

f_p =

.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%

N_e -Number of planets per star that could sustain life like earth

N_e =

.33 .5 .67 1 1.5 2 3 4 5

f_l - Percentage of those planets where life actually develops

f_l =

.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%

f_i - Percentage of planets from f_l with intelligent life

f_i =

.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%

f_c - Percentage of planets from f_i where technology develops

f_c =

.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%

L - "Lifetime" of communicating civilizations (years)

L =

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%

R * f_p * N_e * f_l * f_i * f_c * L = N

N - Number of communicative civilizations

N =