99.999% availability allows about 5 minutes 15 seconds of downtime per year, or about 26 seconds per month. That budget includes every failed deploy, certificate expiry, DNS misconfiguration, and provider incident in the request path. One regional outage lasting an hour consumes more than 11 years of five-nines budget.

This guide explains which architecture tiers can reach that number, which cannot, and why. It covers the published SLAs behind each tier, the difference between an SLA and the availability your users measure, and the October 20, 2025 AWS us-east-1 outage as a worked example. If you run in a single region on a single cloud today, it also helps you decide whether you need to go further.

Downtime budgets by availability target

Each additional nine cuts the allowable downtime by a factor of ten.…