Hosting Uptime/Downtime Calculation

While searching for hosting companies for a number of websites I'm working on, I noticed that a lot companies in Spain try to sell 99.5% guaranteed uptime as being very little downtime. Since this seemed like less than the average to me, I figured I might do a little calculation.

guaranteed uptime	max downtime	downtime / week	downtime / month	downtime / year
99%	1%	1.68 hours	7.2 hours	3.65 days
99.5%	0.5%	50.4 minutes	3.6 hours	1.825 days
99.9%	0.1%	10.08 minutes	43.2 minutes	8.76 hours
99.99%	0.01%	1.008 minutes	4.32 minutes	52.56 minutes
99.999%	0.001%	6.048 seconds	25.92 seconds	5.256 minutes
99.9999%	0.0001%	0.6048 seconds	2.592 seconds	31.536 seconds
99.99999%	0.00001%	0.06048 seconds	0.2592 seconds	3.1536 seconds

While most of these scenarios are not really realistic, it is interesting to see what companies are really offering. Reading about the recent Amazon S3 outage makes one want to place things into context. Amazon's goal is to offer 99.9% uptime, which means a maximum of 0.1% downtime and therefore not more than 43.2 minutes of downtime per month. The outage took little more than 2 hours which qualifies it as a 99.5% host this month. That's a long way from the 99.999% they should be offering for cloud computing indeed.