Free tool
SLA Uptime Calculator
Turn an uptime percentage into the downtime it actually allows per day, week, month and year, and compare the common SLA tiers side by side.
What the nines actually cost you
Three nines sounds reassuring until you convert it: 99.9 percent allows nearly forty four minutes of downtime a month, which is one bad reboot. Four nines allows four and a half minutes, which means no unplanned restart and no manual intervention, because a person noticing and responding takes longer than the entire budget.
Every extra nine costs roughly ten times more to deliver, because it removes a whole class of single point of failure. Going from three nines to four is not a tuning exercise, it needs redundant hardware, automatic failover and monitoring that acts without waiting for a human.
Read the exclusions before signing anything. Most hosting agreements exclude scheduled maintenance from the calculation, so a provider can take you offline for hours on a Sunday and still report a perfect month. The number only means something alongside what it does not count.