#bg { position: fixed; top: 0; left: 0; } .bgwidth { width: 100%; } .bgheight { height: 100%; } $(window).load(function() { var theWindow = $(window), $bg = $("#bg"), aspectRatio = $bg.width() / $bg.height(); function resizeBg() { if ( (theWindow.width() / theWindow.height()) < aspectRatio ) { $bg .removeClass() .addClass('bgheight'); } else { $bg .removeClass() .addClass('bgwidth'); } } theWindow.resize(function() { resizeBg(); }).trigger("resize"); });