Uptime

Hosting providers often promise an uptime, for example 99.9 per cent. That sounds like almost always. Convert it to hours and you see what it means in practice.

How to work it out

A year has 365 × 24 = 8,760 hours. Downtime is what is left when you subtract the uptime from 100 per cent. At 99.9 per cent uptime, downtime is 0.1 per cent of 8,760 hours, which is 8.76 hours a year, or about 44 minutes a month. At 99 per cent, it is 87.6 hours a year, more than three and a half days. At 99.99 per cent, it is just under 53 minutes a year.

When the downtime happens matters too

Nine hours spread over a year in short blips at night are barely noticed. Nine hours in one stretch on a Monday morning are noticed by everyone. So also ask when maintenance is done, and who finds out that the site is down. That should be the provider, not a customer ringing you. A CDN can also absorb traffic spikes that could otherwise knock the server over.

Check the promise you have

Find your agreement with your hosting provider, or search their website for "uptime" or "SLA". Convert the percentage to hours with the sum above. Also check what the agreement says you get if they do not keep the promise, and whether it says anything about that at all.