Reliability & Operations
What each “nines” target costs in real downtime (99.9% = 8.8 h/year), how serial dependencies multiply to erode it, and why tail latency — not the average — is what your SLO should track.
Updated August 30, 2026 · 4 min read
| 99% (two nines) | ~3.65 days / year~7.2 h / month |
|---|---|
| 99.9% (three nines) | ~8.76 h / year~43 min / month |
| 99.99% (four nines) | ~52.6 min / year~4.3 min / month |
| 99.999% (five nines) | ~5.26 min / year~26 s / month |
| Serial dependencies multiply | Two 99.9% services in series → ~99.8%every hard dependency lowers the ceiling |
|---|---|
| Redundancy (parallel) | n independent copies → 1 − (1 − a)ⁿhow you claw nines back |
| Error budget | 1 − SLO99.9% → 0.1% budget to spend on releases |
| Track percentiles, not the mean | p50 / p95 / p99 / p99.9the mean hides the tail users feel |
|---|---|
| Fan-out amplifies the tail | A request hitting 100 services sees each service’s p99one slow dependency dominates |