SLA & Uptime Calculator
How much downtime 99.9% - or any other number - actually allows, the reverse for an outage you've already had, and whether your monitor checks often enough to tell.
| Period | Allowed downtime | Uptime |
|---|
Months and quarters are averages of a 365.25-day year (a month is 30.44 days). Your SLA may count calendar months instead - with 30 days, 99.9% allows 43m 12s; with 31, 44m 38s.
Can your monitor even see it?
A monitor only knows what it samples. Between two checks, a site can go down and come back unseen - so the check interval sets the shortest outage you can measure.
From downtime to uptime
Had an outage? See what it did to the number.
The nines, at a glance
| Uptime | Per year | Per month | Per week | Per day |
|---|---|---|---|---|
| 90% | 36d 12h | 3d 1h | 16h 48m | 2h 24m |
| 95% | 18d 6h | 1d 12h | 8h 24m | 1h 12m |
| 99% | 3d 15h | 7h 18m | 1h 40m | 14m 24s |
| 99.5% | 1d 19h | 3h 39m | 50m 24s | 7m 12s |
| 99.8% | 17h 31m | 1h 27m | 20m 10s | 2m 53s |
| 99.9% | 8h 45m | 43m 50s | 10m 5s | 1m 26s |
| 99.95% | 4h 22m | 21m 55s | 5m 2s | 43s |
| 99.99% | 52m 36s | 4m 23s | 1m | 9s |
| 99.995% | 26m 18s | 2m 11s | 30s | 4s |
| 99.999% | 5m 16s | 26s | 6s | 0.864s |
How the numbers are worked out
Allowed downtime is (1 - uptime) × period. For 99.9% over a year: 0.001 × 365.25 days = 8 hours 45 minutes 58 seconds. Each extra nine divides the budget by ten - which is why the jump from 99.9% to 99.99% (43 minutes a month down to 4 minutes 23 seconds) costs so much more than it sounds.
Why the check interval matters
A monthly 99.99% budget is about 4 minutes 23 seconds. A monitor that checks every 5 minutes - the free tier of most hosted services - has one sample for that whole budget: an outage of four minutes can start just after one check and end just before the next, and the month still reads 100%. The opposite error happens too: one failed check is booked as a full five minutes down, even if the blip lasted ten seconds.
So the rule of thumb: you need several checks inside your downtime budget before your uptime number means anything. For 99.9% (43 minutes a month) a 1-minute interval is plenty; for 99.99% you want 15 to 30 seconds.
Common questions
How much downtime is 99.9% uptime?
About 8 hours 46 minutes a year, 43 minutes 50 seconds a month, 10 minutes a week, or 1 minute 26 seconds a day.
What does five nines (99.999%) mean?
At most 5 minutes 16 seconds of downtime a year - 26 seconds a month. It's a telecoms-grade target; few web services promise it, and fewer measure finely enough to prove it.
Does planned maintenance count?
Depends on the contract. Most SLAs exclude announced maintenance windows, and most count only outages longer than a few minutes, or only from when you report them. Read the definitions before comparing two providers' percentages.
How do I measure my own uptime?
Check the site on a fixed interval and record every change. UpWatchr is a free Windows app that does that for every site, API and server you list, keeps the history, and alerts you the moment one goes down. For a single check right now, there's the HTTP status checker.