box-sizing: border-box 会直接将border 和padding 放到盒子里面。

<!DOCTYPE html>
<html>
<head>
    <title></title>
    <style type="text/css">div{
            border:1px solid #f00;
            width:300px;
            height:300px;
            padding:0 15px;
            box-sizing: border-box;
        }</style>
</head>
<body>
    <div></div>
</body>
</html>