I've got a div#bodyContent of variable width. Inside this I've got a div with a 1px border all the way around (let's call it 'the box'). I want the box including the border to fill out the entire width of #bodyContent regardless of whether the content of the box takes up that much space.

The problem is that my solution of assigning the box a width of 100% seems to be interpreted by Firefox (IE is of no interest since it's local css) as the width of the box without padding or border. Which means that the width of the box including borders becomes 100% plus 2px (left and right border).

I'm not entirely sure that the above interpretation of the box model is correct but if it is, how do I accomplish this?

P.S. I can't change the html/add wrappers/containers etc.