20TB in RAIDZ2 = 16.6TiB usable?

[Schmich]

I have 7x 4TB HDDs in RAIDZ2 which should give 20TB = 18.2TiB however I'm only getting 16.6TiB available. My question is this normal? It seems a lot for extra stuff! And if you're bored you can list in short what the extra space is used for, I'm curious :)

 

[Caesar]

I use the drobo calculator and it shows that your actual size is 25.46 TB and avail for data is 16.94 which looks to be about what you are getting.

 

[Schmich]

Hey thanks for helping out! I just wanted to make sure before I start putting data on it. I'm curious as to what the extra 1.6TiB is for.