Div height 100% excluding header

You can use box-sizing property for this

Check this:

http://jsfiddle.net/Gn8zN/1/

Another simple & best solution

Check this:

http://jsfiddle.net/B8J2H/

Here is an article about this problem. CSS 100% height problem

You can see the example page has a perfect 100% layout what header and footer. It uses relative position and not absolute.

This should work:

http://jsfiddle.net/94JNZ/1/

#content
{
    height: auto;
    width: 100%;
    position: absolute;
    top: 40px;
    bottom: 0;
}