Every hosting SLA is quoted as a percentage — 99.9%, 99.99% and so on — but those numbers only mean something once you translate them into real downtime. The calculator below shows the maximum outage each uptime target allows, per day, week, month and year. Each extra “nine” cuts the allowed downtime by roughly ten times.
"Allowed downtime" is the maximum outage that still meets the SLA — planned maintenance and incidents combined. Each extra nine cuts the budget by roughly 10×.
What the “nines” actually mean
99% (two nines) sounds high, but it permits over three and a half days of downtime a year. 99.9% (three nines) — the most common shared-hosting promise — allows about 8.8 hours. 99.99% (four nines) tightens that to roughly 52 minutes, and 99.999% (five nines) to just over five minutes a year, which usually requires redundant infrastructure and automated failover.
When you read an SLA, check what counts against the number: many providers exclude scheduled maintenance, so the real-world availability you experience can differ from the figure on the page.