“Nines” is shorthand for expressing availability targets by the number of nines in the percentage:
- Three nines — 99.9% ≈ 43 minutes downtime per month
- Four nines — 99.99% ≈ 4.3 minutes per month
- Five nines — 99.999% ≈ 26 seconds per month
Each extra nine cuts allowed downtime by roughly ten times — and usually costs far more to achieve. See exact figures in the uptime/SLA calculator.
