How much downtime does each 'nine' allow, and how do you pick an SLO target?

Summary

Per 30 days: 99% allows about 7.2 hours, 99.9% about 43 minutes, 99.95% about 22 minutes, 99.99% about 4.3 minutes, and 99.999% about 26 seconds. Pick the target from what users need and what the business will pay for, not 'as high as possible'.

Longer Answer

Every extra nine costs much more: redundancy across zones or regions, faster detection and automated recovery, stricter change processes. It is also capped by dependencies: a service cannot be meaningfully more available than the critical services it calls (unless it degrades gracefully), and users' own networks and devices fail far more often than 99.99%.

Practical approach: measure current performance, set an SLO slightly below it, check that missing it would actually upset users, and tighten over time if needed. 100% is the wrong target for nearly everything.

Key Points

• ~43 min/month at 99.9%; ~4 min at 99.99%.
• Each nine costs much more.
• Bounded by dependencies.
• Start from current performance and user needs.