Web Tool Bench Uptime SLA Calculator

The Uptime SLA Calculator on Web Tool Bench converts any uptime percentage into the downtime it permits across five periods, from a day to a year. 99% sounds close to 99.9%, but it allows about 7.2 hours of outage a month against 43.8 minutes. The page explains what each additional nine requires in practice, from a single well-run server at 99.9% to automatic failover at 99.99%, and points out that most hosting SLAs only refund a share of the monthly fee. Useful when comparing hosts or writing your own SLA if you resell hosting. Free, no sign-up, and nothing you enter is stored.