How to: The Nine's of availability
Availability is often quantified by uptime (or downtime) as a percentage of time the service is available. It is generally measured in the number of 9s.

$$
Availability = \frac{Uptime}{(Uptime + Downtime)}
$$

If availability is 99.00% available, it is said to have "2 nines" of availability, and if it is 99.9%, it is called "3 nines", and so on.

| Availability (Percent)   | Downtime (Year)    | Downtime (Month)  | Downtime (Week)    |
| ------------------------ | ------------------ | ----------------- | ------------------ |
| 90% (one nine)           | 36.53 days         | 72 hours          | 16.8 hours         |
| 99% (two nines)          | 3.65 days          | 7.20 hours        | 1.68 hours         |
| 99.9% (three nines)      | 8.77 hours         | 43.8 minutes      | 10.1 minutes       |
| 99.99% (four nines)      | 52.6 minutes       | 4.32 minutes      | 1.01 minutes       |
| 99.999% (five nines)     | 5.25 minutes       | 25.9 seconds      | 6.05 seconds       |
| 99.9999% (six nines)     | 31.56 seconds      | 2.59 seconds      | 604.8 milliseconds |
| 99.99999% (seven nines)  | 3.15 seconds       | 263 milliseconds  | 60.5 milliseconds  |
| 99.999999% (eight nines) | 315.6 milliseconds | 26.3 milliseconds | 6 milliseconds     |
| 99.9999999% (nine nines) | 31.6 milliseconds  | 2.6 milliseconds  | 0.6 milliseconds   |