How to use the uptime calculator
- Choose whether you know a target uptime percentage or a total amount of downtime.
- Enter the exact reporting-period length and select minutes, hours or days. For a 30-day month enter 30 days; for a 31-day month enter 31. A common non-leap year is 365 days.
- For target mode, enter a percentage from 0 through 100. For observed mode, enter downtime and its unit. The unused field is ignored.
- Select Calculate. Editing any field clears the previous result so a value from an earlier period cannot be mistaken for the current one.
Uptime, downtime and the reporting period
Allowed downtime equals reporting-period duration multiplied by (1 − uptime percentage ÷ 100). At 99.9% uptime, the unavailable share is 0.1%, or 0.001 of the period.
When downtime is known, achieved uptime equals (period − downtime) ÷ period × 100. The calculator also shows available time. Downtime cannot be greater than the period because that would make available time negative.
A “month” does not have one hidden duration here. You explicitly enter 28, 29, 30 or 31 days as appropriate. A year can be 365 or 366 days. This makes the denominator visible and lets you use a custom contractual window.
Duration output selects useful units automatically. Very small positive allowances are shown in milliseconds or microseconds instead of rounding to a misleading zero. The percentage display uses a few significant digits when more than two decimals are meaningful.
This percentage is a fraction of elapsed time, not a count of successful requests or healthy servers. Some service agreements use request success, exclude certain events, or combine regions by a weighting rule. In those cases, use this result only as a time-based cross-check and follow the agreement’s own measurement definition for the official figure.
Worked examples
A 99.9% target over exactly 30 days allows 2,592 seconds of downtime, displayed as 43 minutes 12 seconds. Available time is 29 days 23 hours 16 minutes 48 seconds.
If a service has 43.2 minutes of downtime during a 30-day period, achieved uptime is 99.9%. If the same downtime is measured over 31 days, the percentage is different because the denominator is longer.
A 99.9999% target over one day allows about 86.4 milliseconds. The calculator keeps this small positive duration visible rather than displaying 0 seconds.
Scope and common mistakes
- Use the reporting period defined by the SLA or operational review. Do not mix a calendar month, a 30-day rolling window and a billing period.
- The result is simple continuous-time arithmetic. It does not apply maintenance windows, request-based availability, regional weighting, rounding clauses, incident exclusions or service credits.
- Uptime percentage is not a promise of future performance. The tool does not monitor endpoints, collect incident data or connect to a provider.
- In observed mode, combine outage intervals carefully and avoid double-counting overlapping incidents before entering the total.
- Copy saves the displayed result only. Inputs remain in this browser and are not submitted to a monitoring service.
Frequently asked questions
How much downtime does 99.9% allow in 30 days?
It allows 43 minutes 12 seconds when the denominator is exactly 30 × 24 hours.
Why not offer a generic “month” button?
Calendar months differ in length. Entering the number of days exposes the assumption and avoids a hidden 30-day or average-month approximation.
Does planned maintenance count as downtime?
That depends on the applicable SLA or internal definition. This calculator includes whatever total you enter and does not decide exclusions.
Can I calculate uptime from several outages?
Yes, first sum the non-overlapping outage durations for the same reporting period, then enter that total in observed mode.
Is 100% uptime always zero downtime?
Mathematically yes for the entered period. Whether a contract measures or rounds availability that way must be checked in its own terms.