SLA Uptime Calculator
What "three nines" or "five nines" really allows — turn an uptime percentage into the downtime you can have per day, month and year.
How it works
downtime = (1 − uptime%) × length of period
An availability figure is just the slice of time a service may be down. Subtract it from 100%, then multiply by the period to get the downtime budget. Each extra "nine" cuts that budget by roughly ten times: 99% allows over three days a year, 99.9% about nine hours, and 99.999% — the famous five nines — barely five minutes. That is why every nine is dramatically more expensive to engineer than the last.
Worked example
A 99.9% SLA means 0.1% downtime. Over a 365.25-day year that is 0.001 × 525,960 minutes ≈ 526 minutes, or about 8 hours 46 minutes a year — roughly 43 minutes a month.
Frequently asked questions
What does "five nines" mean?
99.999% availability — about 5 minutes 15 seconds of downtime a year. It is a demanding target reserved for critical infrastructure, because it leaves almost no room for maintenance windows or unexpected faults.
Does the SLA include planned maintenance?
It depends on the contract. Many providers exclude scheduled maintenance windows from the uptime calculation, so the real customer-visible downtime can exceed the headline number. Always read what counts as "downtime."
How is uptime actually measured?
Usually as (total time − downtime) ÷ total time over a billing period, often measured by external monitors polling the service. Partial outages and degraded performance may or may not count depending on the agreement.
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This calculator is for educational purposes. Double-check important results before acting on them.