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.

Table of values

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.

See also

• Uptime
• High availability