What does an uptime percentage mean?
Uptime is the share of time a service is available. Hosting companies often promise it in a service level agreement (SLA), usually as a number of “nines”. Each extra nine cuts the allowed downtime by ten times: 99% allows more than 7 hours a month, 99.9% about 44 minutes, and 99.99% about 4 minutes.
Formulas
Downtime = (1 − uptime ÷ 100) × length of the periodA month is counted as one twelfth of 365.25 days (30.44 days)
Worked example
A host that promises 99.9% uptime may be down for up to 43m 50s a month, 1m 26s a day or 8h 45m 58s a year without breaking its promise.
How to use the uptime & SLA calculator
- Enter your figures: uptime.
- Read the result. It updates as you type, so there is no button to press.
- Try other numbers to compare scenarios. Reset brings back the example values.
Getting it right
- Read what the SLA counts: planned maintenance is often left out, and the compensation is usually a small credit, not a refund of lost sales.
- Measure it yourself with an uptime monitor; many have free plans that check every few minutes.
- For a small business site, 99.9% is a sensible minimum. Shops and apps that earn money every hour should look for 99.95% or better.
Frequently asked questions
How much downtime is 99.9% uptime?
About 43 minutes a month, or 8 hours 46 minutes a year.
How much downtime is 99.99% uptime?
About 4 minutes 23 seconds a month, or 52 minutes a year.
Is 100% uptime possible?
No service can guarantee it. Hardware fails and networks have problems; good hosts reduce downtime with redundant servers and fast fixes, but some is unavoidable.