Need help calculating usable space for EXT4 formated drive in Raid5

[Moogle Stiltzkin]

Ok i'm stuck here.

I am planning to get a backup for my QNAP 509 Pro NAS which is equiped with 5 x 1tb Western Digital RE3s in raid5 by the way, however i need to calculate how much usable space i need. My drives are all formated in EXT4.


2 x 2tb in JBOD may not necesssarily be 4tb



This tool supposedly calculates usable space for raid modes, however isn't usable space different depending on the format e.g. NTFS, EXT4 etc ? So how accurate is this tool :d ?

http://www.ibeast.com/content/tools/RaidCalc/RaidCalc.asp



The tool doesn't calculate for JBOD, can anyone help