i found this as the most cleanest way to explain uptime . This is usually the starting point of designing solutions . What kind of downtime can the business handle , we can then match the appropriate level of design with the required uptime
System uptime can be expressed as three nines, four nines, or five nines. These expressions indicate system uptimes of 99.9 percent, 99.99 percent, or 99.999 percent. To calculate system uptime in terms of hours, multiply these percentages by the number of hours in a year (8,760).
| Uptime level | Uptime hours per year | Downtime hours per year |
|---|---|---|
| 99.9% | 8,751.24 | (8,760 – 8,751.24) = 8.76 |
| 99.99% | 8,759.12 | (8,760 – 8,759.12) = 0.88 |
| 99.999% | 8,759.91 | (8,760 – 8,759.91) = 0.09 |