设置宽度为100％，同时设置边距

I have two sections of a page, a side bar and a main content area. The side bar is 225px wide and I'd like the main content area to be 100% width of the page, minus the 225px for the sidebar. So I set the main content area to have a margin of 225 on the left so that it doesn't overlap the side bar, then I set its width to 100%, but doing this pushes the entire document out by 225pxs on the right, skewing the entire page. How can I set the width to be 100% while also having a margin and not pushing the document out as well? I have:I have two sections of a page, a side bar and a