<b>Q: What does "99.9% uptime" actually allow in downtime?</b>

<b>Q: What does "99.9% uptime" actually allow in downtime?</b>

A: Here's the math you can keep in your head. Per year:
— 99% = about 3.65 days of allowed downtime
— 99.9% ("three nines") = about 8.77 hours
— 99.99% ("four nines") = about 52.6 minutes
— 99.999% ("five nines") = about 5.26 minutes

Every extra nine roughly cuts your allowed downtime by 10x, and your costs go up much faster than that.

The catch most people miss: nines are meaningless without a measurement window. 99.9% per year hides a 9-hour outage. 99.9% per month allows only ~43 minutes. Always pin down whether the SLA is measured monthly or annually before you sign or promise it.

Got a question? Drop it in the comments.