SLA Uptime Calculator - Convert 99.9% Availability to Allowed Downtime
SLA Uptime Calculator
Downtime budgets for availability targets, and combined uptime
Downtime Budget
See exactly how much downtime 99.9% or 99.99% allows per day, month and year.
Both Directions
Turn an outage you had into the availability you actually delivered.
System Availability
Multiply dependencies in series, or see how redundancy in parallel adds nines.
Private
Calculated in your browser.
What the Nines Mean
Availability is the share of time a service works. "Three nines", 99.9%, allows about 8 hours 46 minutes of downtime a year or 43.8 minutes a month; "four nines", 99.99%, allows only 52.6 minutes a year. Months here are a twelfth of an average Gregorian year (30.44 days), which is how most SLA calculators and cloud providers count; a 31-day calendar month allows slightly more.
When a request needs several components that all have to work, their availabilities multiply: three services at 99.95%, 99.9% and 99.99% give about 99.84% together, lower than any of them. Redundant copies in parallel do the opposite: two independent 99% servers give 99.99%. Real failures are rarely fully independent, so treat parallel results as an upper bound.
Key Takeaways
- Every nine counts: Each extra nine cuts the downtime budget by a factor of ten.
- Dependencies multiply: A chain is less available than its weakest link.
- Check the contract: SLAs often exclude planned maintenance and measure per calendar month.