The colloquial term nines is used in engineering to indicate reliability or purity (or similar quantitative descriptor). It is preceded by a number indicating the degree of such reliability or purity. For example, 0.999 (or 99.9 percent) pure silver would be 3 nines pure. Or, for example, electricity that is delivered without interruptions (blackouts, brownouts or surges) 99.999 percent of the time would have 5 nines reliability. Reliability can be considered a time-based purity. Also related are the topics of defect rates, Parts-per notation, MTBF and MTTR.
| Nines | Availability | % | Downtime per year | Downtime per month* | Downtime per week |
|---|---|---|---|---|---|
| one nine | 0.9 | 90% | 36.5 days | 72 hours | 16.8 hours |
| two nines | 0.99 | 99% | 3.65 days | 7.20 hours | 1.68 hours |
| three nines | 0.999 | 99.9% | 8.76 hours | 43.2 minutes | 10.1 minutes |
| four nines | 0.9999 | 99.99% | 52.56 minutes | 4.32 minutes | 1.01 minutes |
| five nines | 0.99999 | 99.999% | 5.26 minutes | 25.9 seconds | 6.05 seconds |
| six nines | 0.999999 | 99.9999% | 31.5 seconds | 2.59 seconds | 0.605 seconds |
* For monthly calculations, a 30-day month is used.