I came across a couple of RAID disk space calculators day before yesterday and thought of sharing them. They basically helps you to figure out the usable space you get out of your available disks when configured under a particular RAID level(say RAID 0, RAID 1, RAID 5, etc). It also gives sufficient information about each RAID level like no. of disks it should have etc. Another good thing is, they are available online. So you can connect and calculate anytime. Below are the links.
Let me know have comes across any such interesting tools.