A 99.9% uptime guarantee certainly sounds like it’s reliable enough—that is, until you do the math! Because over the course of a year, 99.9% uptime—or “three nines”, in industry terms—adds up to more than eight hours of unplanned downtime. In fact, when a provider offers you 99.9% uptime, here’s what they’re also telling you: “You should imagine two days over the next year when your email will be unavailable for four-hour stretches at a time.”

Here's the break down on how long your web site will be down:
99% ("two nines") 3.65 days
99.5% ("two nines five") 1.83 days
99.8% ("two nines eight") 17.53 hours
99.9% ("three nines") 8.77 hours - YOUR SITE IS OFF LINE
99.95% ("three nines five") 4.38 hours
99.99% ("four nines") 52.60 minutes
99.995% ("four nines five") 26.30 minutes
99.999% ("five nines") 5.26 minutes
99.9999% ("six nines") 31.56 seconds
99.99999% ("seven nines") 3.16 seconds
99.999999% ("eight nines") 315.58 milliseconds
99.9999999% - 100% ("nine nines") 31.56 milliseconds OFF LINE ONLY MILLLISECONDS
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