Facebook persistent bottom bar

I’m trying to get a similar effect to Facebook’s persistent bottom bar. My situation is a little different:


#barLeft {
   display: fixed;
   width: 150px;
   height: 50px;
   left: 0;
   bottom: 0;
}

#barRight {
   display: fixed;
   width: 100%;
   height: 50px;
   left: 150px;
   bottom: 0;
}

Somehow I need to set barRight’s width to 100% - 150px so it doesn’t extend beyond the screen, but expressions aren’t allowed in CSS. So how can this be done?