About Uptime Calculator

Calculate system uptime and availability percentages with nines notation. Part of the DevTools Surf developer suite. Browse more tools in the Calculators collection.

Use Cases

• Calculate allowed annual downtime for a given availability target to communicate to stakeholders.
• Determine whether a recent outage duration has exceeded the monthly SLA allowance.
• Compare availability tiers when selecting a cloud provider or hosting plan.
• Calculate the financial impact of downtime using revenue-per-minute to justify reliability investments.

Tips

• Calculate downtime budget in minutes per month, not just as a percentage — 99.9% sounds reliable but means 43.8 minutes of allowed downtime per month, which fails some use cases.
• Distinguish between availability (uptime / total time) and reliability (mean time between failures) — a system can be highly reliable (rare failures) but have low availability if failures take a long time to recover from.
• Planned maintenance counts against uptime in most SLA definitions unless a maintenance window exclusion is explicitly included in the contract.

Fun Facts

• The phrase 'five nines' (99.999% availability) became a marketing standard in the telecom industry in the 1990s, where AT&T and similar carriers advertised it for voice network reliability. It translates to 5.26 minutes of downtime per year.
• Amazon S3's actual historical availability in the US East region has exceeded 99.99% in most years, despite the SLA only guaranteeing 99.9% — demonstrating that SLAs are contractual minimums, not operational targets.
• The first time the term SLA (Service Level Agreement) appeared in print was in a 1988 paper by Sturm, Morris, and Jander on IT service management — making the concept only about 35 years old.

FAQ

What availability percentage is appropriate for my service?

99.9% (43.8 min/month) is adequate for internal tools and non-critical services. 99.95% (21.9 min/month) is standard for business-critical services. 99.99% (4.4 min/month) is required for financial and safety-critical systems. The cost of achieving each additional nine increases exponentially.

Does 99.9% uptime mean the service is available 99.9% of the time?

It means downtime is limited to 0.1% of time (43.8 min/month). But this is measured against the contractual SLA definition — some SLAs exclude planned maintenance, specific geographic regions, or errors below a success rate threshold.