Uptime and SLA Downtime Calculator
Enter an availability percentage such as 99.9 to see how much downtime it allows per day, week, month and year. Or go the other way: enter an amount of downtime and the period, and get the availability percentage it equals. Everything runs in your browser.
The formula
Allowed downtime = length of the period x (100 minus availability) / 100. For 99.9% over a 30-day month: 2,592,000 seconds x 0.1 / 100 = 2,592 seconds, which is 43 minutes 12 seconds. The reverse is availability = 100 x (period minus downtime) / period.
Period lengths used
- Day: 24 hours (86,400 seconds). Week: 7 days (604,800 seconds).
- Default basis: a month is 30 days (2,592,000 seconds) and a year is 365 days (31,536,000 seconds). Google's availability table in the SRE book gives the same figures, for example 8.76 hours a year and 43.2 minutes a month at 99.9%.
- Average basis: choose it in the calculator to use the average Gregorian year of 365.2425 days (31,556,952 seconds) and a month of one twelfth of that (2,629,746 seconds, about 30.44 days).
- Calendar months really run from 28 to 31 days, so a provider that measures one calendar month allows a little more or less than the 30-day figure. Check how your own agreement defines the measurement period.
Downtime at common targets (30-day month, 365-day year)
| Availability | Per day | Per week | Per month | Per year |
|---|---|---|---|---|
| 99% | 14m 24s | 1h 40m 48s | 7h 12m | 3d 15h 36m |
| 99.5% | 7m 12s | 50m 24s | 3h 36m | 1d 19h 48m |
| 99.9% | 1m 26.4s | 10m 4.8s | 43m 12s | 8h 45m 36s |
| 99.95% | 43.2s | 5m 2.4s | 21m 36s | 4h 22m 48s |
| 99.99% | 8.64s | 1m 0.48s | 4m 19.2s | 52m 33.6s |
| 99.999% | 0.864s | 6.048s | 25.92s | 5m 15.36s |
Reading an SLA
- An availability percentage only means something together with its measurement window and its definition of downtime. Providers publish their own, so read the service level agreement itself.
- Components in series multiply: two services that are each 99.9% available give 99.9% x 99.9% = 99.8001% if both must work.
- Redundant components in parallel fail together less often: two independent 99% systems, either of which is enough, give 100% minus (1% x 1%) = 99.99%. This assumes the failures really are independent.
Study and related tools
Availability, redundancy and service levels appear on CompTIA Server+, Cloud+ and Network+ and in ITIL. Try the free Server+ SK0-005 practice test, the free Cloud+ CV0-004 practice test or the free ITIL 4 Foundation practice test. Related: the RAID calculator, the RAID levels cheat sheet and the free tools page.
Uptime calculator FAQ
How much downtime is 99.9% uptime?
8 hours 45 minutes 36 seconds a year, 43 minutes 12 seconds a month (30 days), 10 minutes 4.8 seconds a week and 1 minute 26.4 seconds a day.
How much downtime is 99.99% uptime?
52 minutes 33.6 seconds a year and 4 minutes 19.2 seconds a month (30 days).
What are the nines?
A shorthand for the number of 9s in the percentage: 99.9% is three nines, 99.99% is four nines and 99.999% is five nines. Each extra nine cuts the allowed downtime by a factor of ten.
Does 100% uptime mean zero downtime?
Yes, 100% allows none, and the calculator shows 0s for every period.
Period lengths follow the Google SRE book availability table (sre.google/sre-book/availability-table), checked on 8 November 2026. The tool was tested against 15 hand-calculated cases and thousands of random cases computed with exact fractions in Python.