How much space does raidz2 'eat'?

[Christian Baer]

Good evening dear users! :-)

I just installed my first file server with zfs/zpool. Until now I only
ever used UFS.

My pool consists of 7 HDDs of the type WDC WD40EFRX-68WT0N0. smart
tells me the user capacity is: 4,000,787,030,016 bytes.

After creating a raidz2 pool, I get this:

Filesystem       Size    Used   Avail Capacity iused ifree %iused  Mounted on
/dev/ufs/root    992M    491M    421M    54%    2.2k  129k    2%   /
devfs            1.0K    1.0K      0B   100%       0     0  100%   /dev
/dev/ufs/var      34G    1.1G     30G     4%    2.1k  4.7M    0%   /var
/dev/ufs/usr      58G    6.0G     47G    11%    269k  7.6M    3%   /usr
arc1              16T    192K     16T     0%       7   35G    0%   /arc1

Notes:
#1 I did not use phyiscal drives but geli-providers. I want an
encrypted pool.
#2 This pool is mainly for cold storage. I do not need extremely high
performance, but I'd rather optimize it for space.

Now I know that while WD works with kB, FreeBSD works with KiB (factor
1024). However, if I break this drive down to that and multiply that by
5, I get 18.19TiB, while df gives me 16TiB. Sure, there is some
overhead and all, but certainly (hopefully) not 2 whole TiB! That would
be more than 10%.

Is this normal or am I missing something?

Regards,
Chris