9

The nine is a logarithmic unit of availability of a system, defined as the negative of the base 10 logarithm of the unavailability. For example, a system available 99 percent of the time has 0.01 (1 percent) unavailability, or -log₁₀(0.01) = 2 nines.

Many computer and communications facilities strive toward five nines (99.999 percent) availability, which implies a total downtime of no longer than five minutes per year.