How to calculate total width of text including the left and right bearing

I am trying to calculate the total width of a span in HTML including both the left and right bearings of the first and last characters. I have already tried Javascript's offsetWidth and jQuery's width & outerWidth functions, but neither of them return the correct value I am seeking. Here is an example of what I mean. Running in Firefox the calculated width is 135px, while measuring with the Web Developer plug-in gives an actual width of 141px.

Edit

Here is what I've tried, and here are the values it gives me: offsetWidth = 135 jQuery.width () = 135 jQuery.outerWidth() = 135

None of them are calculating the 6px overhang on the 'f' (which would make the width 141).