Password length parameters are related by the expression *P* = *L*R* /*S* , where *P* is the probability that a password can be guessed in its lifetime, *L* is the maximum lifetime a password can be used to log in to a system, *R* is the number of guesses per unit of time, and *S* is the number of unique algorithm-generated passwords (the password space).

The degree of password security is determined by the probability that a password can be guessed in its lifetime.