SGS3 and initial memory

Hi. Newby here. Got SGS3 through my AT&T acct. Did a good bit of homework before I made final decision, so I was in love before it ever came outta the box! Advertised as having internal 16g memory was the catch for me. But.. I've been looking at my storage data and can't seem to find 16g in there. I know initial setup/installation of android package & all the bells and whistles obviously takes some of that 16g. But how much? 1g - 2g - - I assume more than a desktop CPU @ startup 'before' it's put into the box. (See... I told you I don't know.) So can someone tell me where I should be looking for that 16g of memory? And, how is memory measured in this instance? I'm sure it's not like filling the gas tank with so many gallons. I'm bugged because I was sold on 16g of memory and outta the box - before I loaded my SIM and sd data it was 12.1. Am I just acting the fool, or did they just not fill 'er up?