What the "nines" mean
- 99% (two nines) — up to ~3.65 days of downtime per year.
- 99.9% (three nines) — up to ~8.77 hours per year.
- 99.95% — up to ~4.38 hours per year.
- 99.99% (four nines) — up to ~52.6 minutes per year.
How to measure honestly
Availability measured by pinging from a single location inflates the result: a service may respond, but with errors or only in some regions. Reliable measurement relies on probes from multiple locations, verifying response content (not just a 200 status), and complementing synthetic monitoring with data from real users.
Example
A 99.9% SLA gives a budget of ~8.77 h of downtime per year. After a 4 h outage in Q1, ~4.77 h remain for the rest of the year — the error budget in practice.
Example
Annual downtime budget: ~8 h 46 min (99.9%)
Q1 outage: 4 h 00 min
Remaining budget (Q2–Q4): ~4 h 46 min