outerWidth(), IE and percentage

outerWidth(), IE and percentage

So I don't know why, but it seems outerWidth() isn't working properly for me. I have this <div> that I give a width of 70% inside a 800px <div>. The first <div> should then have a width of 560px.

So I'm trying to get the computed width of my element, and I thought outerWidth was supposed to do it since in the documentation it says: 
Returns the width of the element, along with left and right padding, border, and optionally margin, in pixels.
For some reason however, it refuses to output the width in pixels, it will only give me the "70" of my 70%. This behaviour happens on IE8, IE9, Firefox, Chrome and Opera... 

Anyone got a clue what's happening?