Make a div center of viewport - Horizontally and vertically [duplicate]

Question

#wrapper
{
 width:500px;
 height:500px;
 margin:0 auto;
 background:#f7f7f7;
 position:absolute;
 left:50%;
 top:50%;
 margin-left:-250px;
 margin-top:-250px;
}

Demo: http://jsfiddle.net/fJtNQ/

Why is this working?

Well, basically you have an absolute positioned element inside the dom. It means that you can position it wherever you want and if you don't have a relative positioned element as parent, left and top will be the distance from the document's left/top origin.

Assigning left:50% and top:50% enables this element to be positioned always in the center of the screen, but in the center you will find the top left corner of the element.

If you have fixed width/height, you can easily 'translate' the point in the center to be actually the center of the wrapper div by giving negative margin-left and margin-top (therefore with the help of some extremely easy basic math their values will be -(width/2) and -(height/2))

EDIT

You can also easily center by using flexbox, plus (a big one) you don't need to specify w/h, i.e.:

body { /* can also be whatever container */
  display: -webkit-flex;
  display: flex;
  -webkit-align-items: center;
  align-items: center;
  -webkit-justify-content: center;
  justify-content: center;
}

#wrapper {
  /* whatever style u want */
}

demo: https://jsfiddle.net/00utf52o/

@stecb's answer works fine if you're working with fixed width/height.

To position a div or an image vertically at the center in all viewports and make it responsive, use this:

#elem-to-center {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}

I just replaced margin-left and margin-top with transform.

If you want a margin surrounding the element, use this:

#outer-elem {
    width: 100%;
    height: 100%;
    padding: 30px;
    background: #fafafa;
    position: relative;
}
#elem-to-center {
    width: calc(100% - 60px);
    max-height: calc(100% - 60px);

    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}

JSFiddle | Source

I found this useful even in responsive views.

Here is a solution for those who need to center a child div relative to viewport and not "relative to parent div". Other solutions mentioned here do not work in this case as they tend to make the child div center relative to the parent div.

<div id="container">
  <div id="wrapper">Always center, No matter the parent div's position</div>
</div>

#wrapper {
 width:300px;
 height:300px;
 margin:0 auto;
 background:green;
 position:fixed;
 left:50%;
 top:50%;
 margin-left:-150px;
 margin-top:-150px;
}

#container {
  display: block;
  position: absolute;
  width: 400px;
  height: 400px;
  border: 1px solid red;
  bottom: 0;
  left: 0;
}

demo here http://jsbin.com/yeboleli/2/edit?css,output

I use jQuery

$(window).resize(function(){

    $('.className').css({
        position:'absolute',
        left: ($(window).width() - $('.className').outerWidth())/2,
        top: ($(window).height() - $('.className').outerHeight())/2
    });

});

// To initially run the function:
$(window).resize();

The simplest and the cleanest way to do this would be by setting the height, margin (and width, if you want) using viewport sizes.
Say you want the height of you container to occupy 60% (60vh) of the screen, you can divide the rest (40vh, since total height of the viewport = 100vh) equally between the top and the bottom margin so that the element aligns itself in the centre automatically.
Setting the margin-left and margin-right to auto, will make sure the container is centred horizontally.

.container {
         width: 60vw; /*optional*/
         height: 60vh;
         margin: 20vh auto;
         background: #333;
 }

<div class="container">
</div>