$( 'span' ).animate({ 'width': 300 }, { duration: 1000, step: function(now, fx) { // Change color to red when the animation is halfways done if( fx.pos > 0.5 ){ $( this ).addClass( 'red' ); } // Show how much of the animation has been completed $( fx.elem ).text(Math.floor(fx.pos * 100) + '%'); } });