Nines are an informal logarithmic notation for proportions very near to one or, equivalently, percentages very near 100%. Put simply, "nines" are the number of consecutive nines in a percentage such as 99% (two nines)[1] or a decimal fraction such as 0.999 (three nines). Their common uses include grading the purity of materials, or measuring the availability of a service.
Nomenclature
The nines are a count of the leftmost digits 9 that appear in a proportion. For example, 90% would be described as "one nine"; 99% as "two nines"; 99.9% as "three nines"; and so forth.
However, there are different conventions for representing inexact multiples of 9. For example, a percentage of 99.5% could be expressed as "two nines five" (2N5, or N2.5)[2] or as 2.3 nines,[citation needed] following from the logarithm definition.
A percentage of 100% would, in theory, have an infinite number of nines – though, in the context of purity of materials, 100% is virtually unachievable.[3]
| Percent | Permille | Nines | ||
|---|---|---|---|---|
| 90% | 900 | 1 nine | 1N | N1.0 |
| 99% | 990 | 2 nines | 2N | N2.0 |
| 99.9% | 999 | 3 nines | 3N | N3.0 |
| 99.95% | 999.5 | 3 nines 5 | 3N5 | N3.5 |
| 99.97% | 999.7 | 3.5 nines[citation needed] | ? | ? |
| 99.99% | 999.9 | 4 nines | 4N | N4.0 |
| 99.995% | 999.95 | 4 nines 5 | 4N5 | N4.5 |
| 99.999% | 999.99 | 5 nines | 5N | N5.0 |
| 99.9999% | 999.999 | 6 nines | 6N | N6.0 |
The number of nines of a proportion x is:[4]
Uses
Precious metals
The exact purity of very fine precious metals such as platinum, gold and silver can be of great interest. Based on the system of millesimal fineness, a metal is said to be one nine or one nine fine if it is 900 fine, or 90% pure. A metal that is 990 fine is then described as two nines fine and one that is 999 fine is described as three nines fine. Thus, nines are a logarithmic scale of purity for very fine precious metals. Similarly, percentages ending in a 5 have conventional names, traditionally the number of nines, then "five", so 999.5 fine (99.95% pure) is "three nines five", abbreviated 3N5.
Canada's Big Maple Leaf, a coin made of gold at 5N (99.999%) purity, stands as the purest gold coin ever minted, anywhere.[5] The purest gold ever achieved was reportedly produced at the Perth Mint in 1957, at "almost six nines" (99.9999%) purity, as measured by the Worshipful Company of Goldsmiths of London.[6]
Gases
The nines scale is also used in other contexts, such as describing the purity of gases. The purity of a gas is an indication of the ratio of it to other gases in its mixture, as measured by volume. Thus, a high purity refers to a low amount of other gases, or impurities. Gases of higher purity are in many contexts considered to be of better quality and are usually more expensive.
The purity of a gas is generally expressed as a grade prefixed with the letter N (rather than postfixed), indicating the "number of nines" in the percentage or decimal fraction. For example, a N2.0 gas is 99% (two nines) pure and 1% impurities by volume; a N6.0 gas is 99.9999% (six nines) pure, with 1 part per million (1 ppm or 1 vpm, volume per million) impurities.[7]
Intermediate values indicate the digit following the last nine. For example, N4.6 estimates a purity level of 99.996% (four nines followed by a six).[7] An alternative representation uses the common logarithm: for example, a gas which is 99.97% pure would be described as N3.5, since log10(0.03%) = −3.523.[citation needed]
System availability
Nines are used in a similar manner to describe computer system availability. In this context, a "one nine" (90%) uptime indicates a system that is available 90% of the time or, as is more commonly described, unavailable 10% of the time – about 72 hours per month.[8] A "five nines" (99.999%) uptime describes a system that is unavailable for at most 26 seconds per month.[8]