var headheight = document.getElementById('head_container').offsetHeight;
var contentheight = document.getElementById('content_container').offsetHeight;
var footheight = document.getElementById('foot_container').offsetHeight;
var optimalheight = viewportheight - (headheight+footheight+50);

if (contentheight < optimalheight) {
  document.getElementById('content_container').style.height = optimalheight + 'px';
}