Uptime guarantees need a standardized measurement to compare systems and set meaningful SLAs between providers and customers.
Availability is quantified by uptime percentage — “number of 9s” (99.9% = three 9s, 99.99% = four 9s). Each additional 9 represents a tenfold reduction in allowed downtime.
- Availability is calculated as uptime divided by total time over a measurement period.
- 99.9% (three 9s) permits approximately 8 hours 45 minutes of downtime per year.
- 99.99% (four 9s) permits approximately 52 minutes of downtime per year.
- 99.999% (five 9s) permits approximately 5 minutes of downtime per year.
- Each additional 9 requires exponentially more redundancy and operational discipline.
- Measured in “number of 9s” — 99.9%, 99.99%, 99.999%
- Each additional 9 is a 10x reduction in allowed downtime
- Three 9s = 99.9% uptime
- Four 9s = 99.99% uptime
- Five 9s = 99.999% uptime
- Built from: Availability in Parallel vs Sequence — how component availability combines to produce overall nines
- Related: Active-Passive Failover — failover pattern increases achievable nines
- Related: Active-Active Failover — active-active improves achievable nines
- Related: Horizontal Scaling — adding nodes increases availability through redundancy
- Nines are calculated over a full year — a single prolonged outage can blow through the entire budget
- “Five 9s” is extraordinarily difficult in practice, requiring redundant everything (power, network, servers, data centers)
- Partial outages (degraded but not down) are often excluded from SLA calculations, masking real availability