Five nines (99.999%) is a high-availability standard that allows no more than approximately 5 minutes and 15 seconds of total downtime per year, or about 26 seconds per month. It is one of the most demanding uptime targets and is typically reserved for mission-critical infrastructure like financial systems, emergency services, and core cloud platform components.
Achieving five nines requires extensive redundancy at every layer (network, compute, storage, application), automated failover with near-zero switchover time, geographic distribution across multiple data centers or regions, exhaustive testing including chaos engineering, and a highly mature incident management process.
Most web applications and SaaS products target three nines (99.9%, ~8.77 hours/year downtime) or four nines (99.99%, ~52.6 minutes/year downtime), which provide an excellent user experience while being more achievable and cost-effective. The jump from four nines to five nines typically requires an order-of-magnitude increase in engineering investment.
