A few days ago I met with Steve Marx, of Azure fame, and he mentioned how people often misunderstand SLAs percentages. So I posted this quiz on twitter:

image

The answer is: 99.99864% (or close)

Why?

One 9 = 99.9% = 100 – 1/10 = 1 – 1/(10^1)

Two 9’s = 99.99% = 100 – 1/100 = 1 – 1/(10^2)

Three 9’s = 99.999% = 100 – 1/1000 = 1 – 1/(10^3)

So,

Two and half 9’s = 100 – 1/(10^2.5) = 99.99684%

 

Daz Wilkins gets an “approved” :-)

 

Update: Ryan Dunn points out that “three 9s” actually refers to “%99.9” and “%99.999” is “five 9s”. However, because I defined what “one 9” and “two 9s” actually meant for me, the quiz stands :-).