How this calculator works
Uptime is the share of a period in which a service was working, usually written as a percentage. Subtract the downtime from the period length and divide by the period length. The number of nines is the base-10 logarithm of the failure share, so 99.9 percent uptime is three nines and allows 43.2 minutes of downtime in a 30-day month. Each extra nine cuts the allowed downtime by a factor of ten. The figures here assume a simple yes-or-no definition of working. Real agreements often exclude scheduled maintenance, count partial degradation differently and measure at a specific point, so read the service level agreement rather than assuming the headline percentage.
Formula
uptime = (period length - downtime) / period length. number of nines = -log10(downtime / period length). allowed downtime = (1 - target) x period length. A 30-day month is 2,592,000 seconds and a 365-day year is 31,536,000 seconds.
Worked example
A service is down for 43.2 minutes in a 30-day month. The month has 2,592,000 seconds and 43.2 minutes is 2,592 seconds, so uptime is 1 - 2,592 / 2,592,000 = 0.999, or 99.9 percent, which is three nines. A target of 99.95 percent over the same month allows (1 - 0.9995) x 2,592,000 = 1,296 seconds, or 21.6 minutes.
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Frequently asked questions
What does four nines mean?
Four nines is 99.99 percent uptime. In a 30-day month that allows 4.32 minutes of downtime; in a year it allows 52.6 minutes. Each additional nine makes the target ten times harder to meet and usually requires redundancy, automated failover and careful change control.
Is scheduled maintenance counted as downtime?
Often it is excluded by the agreement, and sometimes planned maintenance outside a window is excluded as well. This calculator counts every minute you enter, so exclude maintenance yourself if the target is defined that way.
How is uptime usually measured?
Common methods include periodic health checks from several locations, synthetic transactions and user monitoring. A service can look up to a single check yet be unusable for real users, so the measurement method matters as much as the number.
Why does the month use 30 days?
It is the usual convention for monthly service level budgets and makes the arithmetic repeatable. A real month has 28 to 31 days, so a reported monthly uptime can differ slightly from this estimate.