http://jsfiddle.net/L9Srn/4/show/
Take a look:http://jsfiddle.net/L9Srn/4/
var box= $('#container'),
ratio= 0.363,
height= 200;
box.height(height).width(height*ratio);
I think all you need to understand is that in css 100% means usually parent's dimensions. And is subjective and can change..And yet there is no way to select what that % represents.