My iphone say, 13.3 GB and my available is 12.0GB?

My iphone saying my capacity is 13.3GB and my available is 12.0GB. What's my GB for my Phone, is a 8, 16, 32?

It's 16GB, you are getting lower space because it's the conversion from KB to GB.
Also, it's the space for software that you can't use.

That's normal, it uses the extra space for the formatting and files data.