I measured 16.5cm, the apple measurement tape gave me a 7. I went initially for a 6 with sololoop, it fit alright, but after some use it just felt too snug/tight, ordered a 7 and it fits much better (even after a months use).
I then tried a braided loop size 6, and it was too damn tight, exchanged it for a 7, and it fits much better. Though I did return it as, imo, it feels cheap and crap.
so, I recommend to follow apples measurement tape. period.