May be a silly question, but does the site width include any padding you have on the left and right.
So for example if I had a main content container that was 100px, which had empty padding on the left and right of 10px, would my site be 100px or 120px wide ?
Yep, Ralph has it correct, this segment of the SitePoint CSS Reference may be worth reading:
Hi invalidCharacter, welcome to SitePoint!
The total width of an element is the set width (if there is one) plus any left/right padding, margin and borders.
So a div of width 100px with 10px left and right padding, 2px left and right border, and 10px left and right margin would have a total width of 144px.