If you go Settings -> Device -> Data Storage....
You'll see that there's a few places where your storage has been partitioned:
Installed apps 4.2Gb
Application data 2.1GB
User data 8-10GB. (on 64GB it says 57.7GB so 6.3Gb - 6.6Gb set aside)

So you should lose 6.3GB because it's already been set aside for the OS/apps/user data to be stored, the rest is yours for storage like music/videos etc, that MyDocs folder essentially.
I don't believe there's a single phone on the market advertised at 16Gb, where you actually have the full 16Gb available for storage, although the numbers vary by OS etc.

I believe there's ways to change the partition sizes but I'd personally stay away from that jazz.