Uptime calculator

Turn an uptime percentage or an SLA into the downtime it allows per day, week, month and year, or work back from the outages you had to the uptime they mean.

What do you know?
Common SLAs

With more servers

Assumes the servers fail independently: different hosts, ideally different locations or providers.

Downtime allowed per month 43m 50s 99.9% uptime leaves room for 8h 46m of downtime a year.
Day
1m 26s
Week
10m 5s
Month
43m 50s
Quarter
2h 11m
Year
8h 46m
Together (2)
99.9999%
Downtime a year, together
31.6s

A second small server usually buys more uptime than a better SLA on one big one. From €4.48 a month today, among 1,850 servers.

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How it is calculated

Downtime is the part of a period the uptime percentage leaves out. A month here is a twelfth of an average year (30.44 days), the length most SLAs use when they say “per month”; a quarter is three of those.

downtime = period × (1 − uptime ÷ 100)
uptime = (period − downtime) ÷ period × 100
together = 1 − (1 − uptime)ⁿ  for n servers, any one enough

The last line is why redundancy works: two servers that are each down 0.1% of the time are both down at once only 0.0001% of the time, if they fail for unrelated reasons. A load balancer, DNS failover or a database replica is what makes “any one is enough” true.

Example

A 99.95% SLA over a month: 43,830 minutes × 0.0005 = 21.9 minutes. An outage of 22 minutes or less in a month is within the promise.

Common uptime levels

UptimeDayMonthYear
99%14m 24s7h 18m3d 16h
99.5%7m 12s3h 39m1d 20h
99.9%1m 26s43m 50s8h 46m
99.95%43.2s21m 55s4h 23m
99.99%8.64s4m 23s52m 36s
99.999%0.86s26.3s5m 16s

Questions

How much downtime is 99.9% uptime?

About 43 minutes a month, 8 hours 46 minutes a year, or 1 minute 26 seconds a day. 99.99% allows a tenth of that: about 4 minutes 23 seconds a month.

What uptime do VPS and cloud providers promise?

Most promise between 99.9% and 99.99% a month. An SLA is a refund promise, not a guarantee: when the provider misses it you usually get a credit for part of that month’s fee, and often only if you ask for it within a few weeks.

Does a 99.99% SLA mean my site will be up 99.99% of the time?

No. The SLA covers the provider’s part: the host, power and network. Your own software, a full disk, an expired certificate or a bad deploy are outside it, and they cause most outages. A second server in another location does more for real uptime than a higher SLA on one.

How do I work out uptime from downtime?

Divide the time the service was up by the length of the period. Two hours of downtime in a 30-day month (720 hours) is (720 − 2) ÷ 720 = 99.72% uptime.