Four Nines SLA
99.99% Uptime Calculator

"Four nines" refers to 99.99% availability - the percentage contains four 9 digits. It is a common target for high-traffic production systems that need significantly less downtime than three nines allows.

99.99% SLA allows about 4 minutes and 19 seconds of downtime per month (using a 30-day month), or about 52 minutes and 34 seconds per year.

Payment processing, high-traffic e-commerce, real-time collaboration tools, and core SaaS APIs commonly target four nines, since an outage directly and immediately costs revenue or breaks a dependent customer workflow.

Four nines allows roughly 10 times less downtime than three nines: about 52 minutes per year versus about 8 hours 45 minutes. The two percentages differ by only 0.09 percentage points, but the allowed outage time is dramatically different.

It is very difficult. A single server, single data center, or single point of failure will almost certainly exceed a four-nines annual downtime budget during routine hardware failures, patching, or network issues. Multi-zone or multi-region redundancy is the standard way to reach this tier.