What this calculator does
Uptime targets are usually quoted as a percentage, such as 99.9% or 99.99%, informally known by their count of nines. What that percentage actually means in practical terms, how many hours or minutes of downtime it allows over a year, is far less obvious just from looking at the number.
This calculator converts an uptime percentage straight into downtime figures across a year, month, week and day, so a service level agreement or uptime claim can be read as a concrete amount of allowed outage time rather than an abstract percentage.
The formula
Downtime for a given period is that period's total length multiplied by (1 minus the uptime fraction). A year is treated as 365 days, and a month as 30 days, for a consistent, roughly-average figure across all four periods.
| Term | Meaning |
|---|---|
| Uptime | The percentage of time a service is expected to be available, such as 99.9%. |
| Downtime | The remaining time, calculated as the period length multiplied by (1 minus uptime as a fraction). |
The inputs explained
| Field | What to enter |
|---|---|
| Uptime (%) | The uptime percentage being converted, for example 99.9 for "three nines". |
When to use it
Reading a service level agreement
A vendor SLA promising 99.9% uptime allows a specific, calculable amount of downtime per year, which is worth checking against how much outage your own operations could actually tolerate.
Setting an internal reliability target
Choosing an uptime target for an internal system is easier when you can see the downtime budget it implies in familiar units like hours per month, rather than just a percentage.
Comparing "nines" claims
The jump from 99% to 99.9% to 99.99% sounds small on paper but represents a roughly tenfold cut in allowed downtime at each step, which this calculator makes concrete.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much downtime does each level of uptime allow per year?
The commonly quoted "nines" uptime levels and the downtime per year each one allows.
| Uptime | Downtime per year | Downtime per month |
|---|---|---|
| 99% | 3.65 days | 7.20 hr |
| 99.9% | 8.76 hr | 43.20 min |
| 99.95% | 4.38 hr | 21.60 min |
| 99.99% | 52.56 min | 4.32 min |
| 99.999% | 5.26 min | 25.92 sec |
Questions
What does "three nines" mean?
Three nines refers to 99.9% uptime. The nickname counts the number of nines in the percentage: 99% is "two nines", 99.9% is "three nines", 99.99% is "four nines", and so on, with each additional nine cutting the allowed downtime by roughly a factor of ten.
Why does the calculator use 365 days for a year and 30 for a month?
These are standard round figures used for this kind of conversion. A calendar month is not a fixed length, so 30 days is used as a consistent approximation across all months rather than switching figures depending on which month is meant.
Does planned maintenance count as downtime?
That depends on how a particular SLA defines uptime. Some agreements exclude scheduled maintenance windows from the uptime calculation and some do not, so check the specific terms of any agreement before assuming which is meant.
Can uptime be entered as more than 100%?
No, uptime is a percentage of time and cannot exceed 100%. Enter a value greater than 0% and no more than 100%.
For turning that downtime into a specific clock time rather than just a duration, see the Date and time calculator.