<link href="//maxcdn.bootstrapcdn.com/bootstrap/4.1.1/css/bootstrap.min.css" rel="stylesheet" id="bootstrap-css"> <script src="//maxcdn.bootstrapcdn.com/bootstrap/4.1.1/js/bootstrap.min.js"></script> <script src="//cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script> <!------ Include the above in your HEAD tag ----------> <div class="container"> <div class="out"> <div class="in"></div> </div> </div>

.out{ width:300px; height:300px; background:#000; display:flex; align-items:center; justify-content:center; } .in{ width:100px; height:100px; background:red; }