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, but 99.9% ("three nines") is a common baseline for business services, 99.95–99.99% for serious SaaS platforms, and 99.999% ("five nines", under half a minute of downtime per month) 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.

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.