How this calculator works
The equation multiplies a formation rate by successive fractions, the number of suitable planets and a detectable lifetime. Rate and lifetime both use years, so their time units cancel. The output is an expectation, which can be fractional. Values here are illustrative assumptions, not observations or a probability that a signal will be detected. A steady formation rate and an average lifetime are assumed.
Formula
N = R_star f_p n_e f_l f_i f_c L. A missing factor equals N divided by the product of the other factors.
Worked example
With R = 2 stars/year, f_p = 0.5, n_e = 2, f_l = 1, f_i = 0.1, f_c = 0.5 and L = 1000 years, N = 100.
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Frequently asked questions
Are the defaults a scientific consensus?
No. Especially the biological and technological factors are uncertain scenario assumptions.
Can the answer be below one?
Yes. It is an expected number, not a literal partial civilization.
Why cannot a factor be recovered when another is zero?
Multiplication by zero erases information about the unknown factor. Division cannot recover it uniquely.