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99.9% uptime calculator
99.9% uptime is sometimes called “three nines.” Here is exactly how much downtime that target allows per day, week, month, and year, plus a calculator below to check any other target.
99.9% uptime allows
- Per day
- 1m 26s
- Per week
- 10m 4s
- Per month
- 43m 47s
- Per year
- 8h 45m
“Per month” uses a 365/12-day average month, so twelve months of downtime always add up to one year of downtime. A 28, 30, or 31-day calendar month allows a slightly different figure.
How this is computed
99.9% uptime means the remaining 0.1% of the period is the downtime budget the target allows. We apply that fraction to a day (24 hours), a week (7 days), an average month (365 ÷ 12 days, so twelve months of downtime always add up to one year of downtime instead of drifting with each calendar month’s length), and a year (365 days). No rounding tricks: the table above and the figures on this page come from the exact same formula, computed live.
What this number does not tell you
This is the downtime a 99.9% target permits on paper, not a promise that any particular service is meeting it. Whether a vendor actually stayed inside that budget this month is a measurement question, not an arithmetic one, and it depends on who is doing the measuring and from where. If you have a real SLA with a vendor and want to know what a shortfall would cost, see the downtime cost calculator.
Want to know if you’re actually hitting 99.9%?
Knowing the budget is one thing; knowing whether you spent it is another. RealUptime Monitor checks your endpoints on a schedule from four live regions (US East, US West, Europe, and Asia Pacific) and reports your real measured uptime against a target like this one, not just an assumption.
Other uptime targets
99% uptime, 99.5% uptime, 99.95% uptime, 99.99% uptime, 99.999% uptime. See the full SLA calculator overview, or all free tools.