Iphone 5s 16 GB only 8 Gb available

Hello Apple Community,

I got myself a Iphone 5s 16GB some time ago and today i did hit the limit of save capacity. Now i did wonder why it was already at the limit because i dont have to much stuff on my phone. So i checked the System data for save capacity and it said i had a capacity of 12 GB. But when i did check in the itunes store on how much i acctually use it just added up to around 8GB. Did anyone else experience a similar problem or anyone who can help me?