Uptime
Nines
Also called three nines, 99.9%.
Nines is shorthand for availability targets by count of nines: 99.9 percent is three nines. Each extra nine cuts allowed downtime by a factor of ten.
How it is measured
Convert to time per year or month. 99 percent is 3.65 days a year, 99.9 percent is 8.76 hours, 99.99 percent is 52.6 minutes, and 99.999 percent is 5.26 minutes. Over a 30-day month those are 7.2 hours, 43.2 minutes, 4.32 minutes, and 26 seconds.
The conversion hides the window and the definition of down. Always say 99.9 percent of what, measured how, over which period.
Worked example
A founder promises four nines for a landing-page generator. Four nines is 4.3 minutes a month. Last month the generator had one 11-minute database failover, which is 99.975 percent availability.
That is three and a half nines, a respectable result, and a missed promise on the pricing page. Moving the promise to 99.9 gives 43.2 minutes, and the 11-minute event used about a quarter of it.
How it differs
Nines is shorthand for the target. Availability is the measured number. Nines excludes how you measure it. Availability excludes the rounded marketing label.
Common errors
Promising five nines on one server. Not stating the window. Rounding 99.85 to three nines. Excluding maintenance without saying so. Ignoring dependencies, since your nines cannot exceed those of the parts you depend on.
In practice
Pick the nine that matches your dependencies. A site on one host and one DNS provider rarely sustains more than three. Write the minutes per month beside the percentage in your internal docs.