Uptime & SLA Calculator.

That the service may be unavailable for at most 0.1% of the time: about 43 minutes per 30-day month, or 8 hours 46 minutes per year. Whether that is acceptable depends entirely on when it happens: 40 minutes at 4 a.m. on Sunday is very different from 40 minutes during your product launch.

There is no fixed definition. 99.9% ("three nines") is the common standard for business services. From 99.95% to 99.99%, it counts as high availability: that level needs redundant infrastructure, often across several data centers, so it usually costs more. 99.999% ("five nines", under half a minute of downtime per month) is for carrier-grade infrastructure. Each extra nine roughly multiplies the engineering cost.

An SLI (indicator) is the measurement itself, for example the fraction of successful requests. An SLO (objective) is your internal target for that indicator, like 99.95%. An SLA (agreement) is the contractual promise to customers, usually less strict than the SLO and backed by penalties such as service credits.

That depends on the contract, and it is one of the most important clauses to read. Many SLAs exclude announced maintenance windows from the calculation, which can make a "99.9%" promise considerably weaker in practice than the raw number suggests.

About 4 minutes 19 seconds per 30-day month, roughly 52 minutes 34 seconds per year and under 9 seconds per day. Select 99.99% in the calculator above to see every period at once.

With the uptime formula: Uptime % = (total time − downtime) ÷ total time × 100. For example, 90 minutes of downtime in a 30-day month: (43200 − 90) ÷ 43200 × 100 = 99.79%. The reverse calculator on this page does this for you.

A lot. 99.9% measured per year allows all 8.8 hours of downtime to happen in one incident. Measured per month, a single incident can cost at most about 43 minutes before the SLA is breached. Monthly windows are stricter and more common in SaaS contracts.

Measure it yourself. Independent, external uptime monitoring records every outage with timestamps, giving you your real availability and the evidence to claim SLA credits. Relying on a provider's own status page alone is not independent measurement.