Q: What does 99.9% uptime actually allow in real minutes?

Q: What does 99.9% uptime actually allow in real minutes?

A: Translate the nines into a budget you can feel:
— 99.9% (three nines) = ~43 minutes down per month.
— 99.95% = ~22 minutes per month.
— 99.99% (four nines) = ~4.4 minutes per month.
— 99% = ~7.2 hours per month, which sounds fine until you say it out loud.

That per-month "error budget" is the useful framing: it's how much downtime you're allowed to spend. One bad deploy that takes you offline for 20 minutes burns nearly half a three-nines month in a single afternoon.

Follow-up worth answering now: a provider quoting 99.9% "annually" can legally have a single 8-hour outage and still hit the number. Always check whether the SLA is measured monthly or yearly before you trust it.

Got a question? Drop it in the comments.