<body>
<div id="box"></div>
<style>
#box{
width:200px;
height:100px;
padding:5px;
border:1px solid red;
margin:10px auto;
}
</style>
<script>
var oBox=document.getElementById(‘box‘);
var cssJson=oBox.currentStyle||getComputedStyle(oBox);//兼容
alert(cssJson.width);//width 200px
alert(oBox.clientWidth);//width+padding 210px
alert(oBox.offsetWidth);//width+padding+border 212px
</script>
</body>
原文：http://www.cnblogs.com/yuqlblog/p/7953713.html