How uptime percentage is calculated
Uptime percentage is the proportion of a time period during which a system was operational and accessible:
Uptime % = ((Total time − Downtime) ÷ Total time) × 100
This is the inverse of availability calculated from downtime — see the Availability Calculator for the reverse direction (uptime % → allowed downtime).
Common uptime calculation periods
Most SLAs and SLOs measure uptime over a rolling 30-day window, though some use calendar months or fixed annual periods. The period you choose affects how forgiving the calculation is — a single 4-hour outage looks much worse in a 1-day window than in a 1-year window.
| Measurement period | Common use |
|---|---|
| Rolling 30 days | Most SaaS SLAs, internal SLOs |
| Calendar month | Billing-aligned SLA credits |
| Annual | Enterprise contracts, infrastructure SLAs |
| Quarterly | Internal reliability reviews |
Annualizing short-period uptime
If you measure 99.5% uptime over a single day, that does not mean your annual uptime will be 99.5% — a single bad day can disproportionately affect a short window. Annualizing (projecting a short measurement period forward across a full year) shows what your uptime would be if the current rate continued, which is useful for trend analysis but should not be used as an actual SLA compliance figure.
Uptime vs availability
Uptime and availability are often used interchangeably, but technically uptime refers to whether the system process is running, while availability refers to whether users can actually use the system successfully. A service can have 100% uptime (server never crashes) while having poor availability (constantly returning errors under load). See What is Availability? for the full distinction.