Uptime & SLA Calculator: calculate allowed downtime

99.9% uptime means a service must be available 99.9% of the time. That allows about 1 minute 26 seconds of downtime per day, 10 minutes 5 seconds per week, 43 minutes 50 seconds per month, and 8 hours 45 minutes 58 seconds per year. This level is often called “three nines.”

Availability in % = (total time − downtime) ÷ total time × 100. Conversely, allowed downtime = period × (100 − availability) ÷ 100. With 30 minutes of downtime in a month averaging 30.44 days, that works out to around 99.93%. Measure the downtime itself using external monitoring, such as ZAP-Hosting's free server monitoring, then convert it here in “Downtime → Availability” mode.

SLA stands for Service Level Agreement, an agreement between a provider and a customer about the promised quality of service. The contract specifies what the provider commits to, such as 99.9% availability. If measured availability falls below that level during the agreed period, it is an SLA breach; the contract sets out what happens next, such as a credit. The calculator converts the promised percentage into allowed downtime. It does not calculate response or resolution times.

The number of nines is shorthand for uptime: two nines means 99%, three nines 99.9%, four nines 99.99%, and five nines 99.999%. Each additional nine divides allowed downtime by ten. Four nines allows about 4 minutes 23 seconds of downtime per month; five nines allows only about 5 minutes 16 seconds in an entire year.

Most providers measure uptime per calendar month; some use a quarter or year. This makes a big difference: 99.9% per month allows at most about 44 minutes of continuous downtime, while 99.9% per year allows a single outage of almost 9 hours. Check your contract to see which period applies. The calculator shows you the figures for a day, week, month, quarter, and year.

That depends on the contract. Many SLAs exclude scheduled maintenance windows, as well as outages caused by the customer, such as through their own software or configuration. The calculator shows the pure 24/7 math, without such exceptions or business-hour restrictions.

The SLI (Service Level Indicator) is the measured value, such as actual uptime. The SLO (Service Level Objective) is the internal target, such as 99.95%. The SLA (Service Level Agreement) is the contract with the customer and specifies what happens if uptime falls below the target, usually a service credit.

The calculator uses average calendar lengths: a year has 365.25 days, and a month is one-twelfth of that, or 30.44 days. This makes the annual figure exactly twelve times the monthly figure. The figure of 43 minutes 12 seconds comes from using a fixed 30-day month, as some other calculators do. Allowed downtime is slightly longer in a 31-day month and slightly shorter in February.