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

Reliability engineering

Reliability engineering is an engineering field, that deals with the study, evaluation, and life-cycle management of reliability: the ability of a system or component to perform its required functions under stated conditions for a specified period of time. It is often measured as a probability of...

 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 (blackout

Power outage

A power outage is a short- or long-term loss of the electric power to an area.There are many causes of power failures in an electricity network...

s, brownout

Brownout (electricity)

A brownout is an intentional drop in voltage in an electrical power supply system used for load reduction in an emergency. The reduction lasts for minutes or hours, as opposed to short-term voltage sag or dip. The term brownout comes from the dimming experienced by lighting when the voltage sags...

s or surge

Voltage spike

In electrical engineering, spikes are fast, short duration electrical transients in voltage , current , or transferred energy in an electrical circuit....

s) 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

Parts-per notation

In science and engineering, the parts-per notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement...

, MTBF and MTTR

Mean time to recovery

Mean time to recovery is the average time that a device will take to recover from any failure. Examples of such devices range from self-resetting fuses , up to whole systems which have to be repaired or replaced.The MTTR would usually be part of a maintenance contract, where the user would pay...

.

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.