How to center an element in the middle of the browser window?

To do this you need to know the size of the element you are centering. Any measurement will do (i.e. px, em, percent), but it has to have a fixed size.

The css will look as follows:

 // Replace X and Y with a number and u with a unit. do calculations
 // and remove parens
.centered_div {
   width: Xu;
   height: Yu;
   position: absolute;
   top: 50%;
   left: 50%;
   margin-left: -(X/2)u;
   margin-top: -(Y/2)u;
}

Edit: This centers in the viewport. You can only center in the browser window using JavaScript. But that might be good enough anyway, since you probably want to display a popup/modal box?

div#wrapper {
    position: absolute;
    top:  50%;
    left: 50%;
    transform: translate(-50%,-50%);
}

This should work with any div or screen size:

.center-screen {
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  text-align: center;
  min-height: 100vh;
}

 <html>
 <head>
 </head>
 <body>
 <div class="center-screen">
 I'm in the center
 </div>
 </body>
 </html>

See more details about flex here. This should work on most of the browsers, see compatibility matrix here.

I surprised that nobody said about position=fixed. It makes exactly what I asked and works in all "human" browsers and IE since 7.

This is completely possible with just CSS-- no JavaScript needed: Here's an example

Here is the source code behind that example:

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>  
<meta http-equiv="content-type" content="text/html;charset=ISO-8859-1">
<title>Dead Centre</title>  
<style type="text/css" media="screen"><!--
body 
    {
    color: white;
    background-color: #003;
    margin: 0px
    }

#horizon        
    {
    color: white;
    background-color: transparent;
    text-align: center;
    position: absolute;
    top: 50%;
    left: 0px;
    width: 100%;
    height: 1px;
    overflow: visible;
    visibility: visible;
    display: block
    }

#content    
    {
    font-family: Verdana, Geneva, Arial, sans-serif;
    background-color: transparent;
    margin-left: -125px;
    position: absolute;
    top: -35px;
    left: 50%;
    width: 250px;
    height: 70px;
    visibility: visible
    }

.bodytext 
    {
    font-size: 14px
    }

.headline 
    {
    font-weight: bold;
    font-size: 24px
    }

#footer 
    {
    font-size: 11px;
    font-family: Verdana, Geneva, Arial, sans-serif;
    text-align: center;
    position: absolute;
    bottom: 0px;
    left: 0px;
    width: 100%;
    height: 20px;
    visibility: visible;
    display: block
    }

a:link, a:visited 
    {
    color: #06f;
    text-decoration: none
    }

a:hover 
    {
    color: red;
    text-decoration: none
    }

--></style>
</head>
<body>
<div id="horizon">
    <div id="content">
        <div class="bodytext">
        This text is<br>
        <span class="headline">DEAD CENTRE</span><br>
        and stays there!</div>
    </div>
</div>
<div id="footer">
    <a href="http://www.wpdfd.com/editorial/thebox/deadcentre4.html">view construction</a></div>
</body>
</html>

How to center an element in the middle of the browser window?

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