||It has been suggested that this article be merged into High availability. (Discuss) Proposed since March 2013.|
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.
In mathematical terms, the quantity of nines is equal to the negative common logarithm of one minus the actual number, i.e. −log10(1−n).
Table of values 
|one nine||10-1||90%||36.5 d||3 d||16.8 h||2.4 h|
|two nines||10-2||99%||3.65 d||7.2 h||1.68 h||14.4 min|
|three nines||10-3||99.9%||8.76 h||43.2 min||10.08 min||1.44 min|
|four nines||10-4||99.99%||52.56 min||4.32 min||60.48 s||8.64 s|
|five nines||10-5||99.999%||5.256 min||25.92 s||6.048 s||864 ms|
|six nines||10-6||99.9999%||31.536 s||2.592 s||604.8 ms||86.4 ms|
|seven nines||10-7||99.99999%||3.1536 s||259.2 ms||60.48 ms||8.64 ms|
|eight nines||10-8||99.999999%||315.36 ms||25.92 ms||6.048 ms||864 µs|
|nine nines||10-9||99.9999999%||31.536 ms||2.592 ms||604.8 µs||86.4 µs|
* For monthly calculations, a 30-day month is used.
For large amounts of 9s, the "unavailability" index is easier to handle, which is also why such is used in e.g. hard disk bit error rates.
See also 
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