Nines (engineering)

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

Nines	Unav.	Availability	Downtime per
			year	month*	week	day
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

• Class of 9s
• Uptime
• High availability
• 0.999...