Five nines of availability means 99.999% uptime — a service level that allows no more than 5 minutes 15 seconds of downtime per year. It is the gold standard for telecom and critical infrastructure but is rarely achievable, or economically justified, for…
Each additional nine reduces allowed downtime by a factor of ten. 99% (two nines) allows 3.65 days of downtime per year, 99.9% (three nines) allows 8.77 hours, 99.99% (four nines) allows 52.6 minutes, and 99.999% (five nines) allows just 5.26 minutes.
Reaching five nines requires multi-region failover, automated incident response, redundant networking, and zero single points of failure. It is expensive — usually 10x to 100x the cost of running at 99.9% — and rarely justified outside of payments, telecom, and life-critical systems.
Most SaaS contracts specify three nines or four nines. Knowing what nines you actually need stops you from over-engineering — and lets you call out vendor SLAs that quietly only commit to 99% (3.65 days a year of permitted outage).
See it in the product: Public status pages.