Center aligning a fixed position div

I'm trying to get a div that has position:fixed center aligned on my page.

I've always been able to do it with absolutely positioned divs using this "hack"

left: 50%; 
width: 400px; 
margin-left: -200px;

...where the value for margin-left is half the width of the div.

This doesn't seem to work for fixed position divs, instead it just places them with their left-most corner at 50% and ignores the margin-left declaration.

Any ideas of how to fix this so I can center align fixed positioned elements?

And I'll throw in a bonus M&M if you can tell me a better way to center align absolutely positioned elements than the way I've outlined above.

Koen's answer doesn't exactly centers the element.

The proper way is to use CCS3 transform property. Although it's not supported in some old browsers. And we don't even need to set a fixed or relative width.

.centered {
    position: fixed;
    left: 50%;
    transform: translate(-50%, 0);
}

Working jsfiddle comparison here.

For the ones having this same problem, but with a responsive design, you can also use:

width: 75%;
position: fixed;
left: 50%;
margin-left: -37.5%;

Doing this will always keep your fixed div centered on the screen, even with a responsive design.

You could use flexbox for this as well.

.wrapper {
  position: fixed;
  top: 0;
  left: 0;
  height: 100%;
  width: 100%;
  
  /* this is what centers your element in the fixed wrapper*/
  display: flex;
  flex-flow: column nowrap;
  justify-content: center; /* aligns on vertical for column */
  align-items: center; /* aligns on horizontal for column */
  
  /* just for styling to see the limits */
  border: 2px dashed red;
  box-sizing: border-box;
}

.element {
  width: 200px;
  height: 80px;

  /* Just for styling */
  background-color: lightyellow;
  border: 2px dashed purple;
}

<div class="wrapper"> <!-- Fixed element that spans the viewport -->
  <div class="element">Your element</div> <!-- your actual centered element -->
</div>

From the post above, I think the best way is

1. Have a fixed div with width: 100%
2. Inside the div, make a new static div with margin-left: auto and margin-right: auto, or for table make it align="center".
3. Tadaaaah, you have centered your fixed div now

Hope this will help.

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

.container-div {position:fixed; left: 0; bottom: 0; width: 100%; margin: 0;}
.center-div {width: 200px; margin: 0 auto;}

This should do the same.

Normal divs should use margin-left: auto and margin-right: auto, but that doesn't work for fixed divs. The way around this is similar to Andrew's answer, but doesn't use the deprecated <center> thing. Basically, just give the fixed div a wrapper.

#wrapper {
    width: 100%;
    position: fixed;
    background: gray;
}
#fixed_div {
    margin-left: auto;
    margin-right: auto;
    position: relative;
    width: 100px;
    height: 30px;
    text-align: center;
    background: lightgreen;
}

<div id="wrapper">
    <div id="fixed_div"></div>
</div

This will center a fixed div within a div while allowing the div to react with the browser. i.e. The div will be centered if there's enough space, but will collide with the edge of the browser if there isn't; similar to how a regular centered div reacts.

Center it horizontally:

display: fixed;
top: 0; 
left: 0;
transform: translate(calc(50vw - 50%));

Center it horizontally and vertically (if its height is same as width):

display: fixed;
top: 0; 
left: 0;
transform: translate(calc(50vw - 50%), calc(50vh - 50%));

No side effect: It will not limit element's width when using margins in flexbox

If you want to center aligning a fixed position div both vertically and horizontally use this

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

If you know the width is 400px this would be the easiest way to do it I guess.

 left: calc(50% - 200px);

This works if you want the element to span across the page like another navigation bar.

width: calc (width: 100% - width whatever else is off centering it)

For example if your side navigation bar is 200px:

width: calc(100% - 200px);

It is quite easy using width: 70%; left:15%;

Sets the element width to 70% of the window and leaves 15% on both sides

I used the following with Twitter Bootstrap (v3)

<footer id="colophon" style="position: fixed; bottom: 0px; width: 100%;">
    <div class="container">
        <p>Stuff - rows - cols etc</p>
    </div>
</footer>

I.e make a full width element that is fixed position, and just shove a container in it, the container is centered relative to the full width. Should behave ok responsively too.

The conventional approach of .center { position: fixed; left: 50%; top: 50%; transform: translate(-50%, -50%); } tries to keep some empty space around the centered element which often causes the centered element's width to not be ideal like being smaller than what it should be.

@proseosoc's answer works well in that scenario and extends the centered element's reach to the end of the viewport sides. The centered element, however, gets centered to the entire viewport including the scrollbar. But if your use case requires centering elements within space without the scrollbar, you can use this modified answer. This approach is also similar to the aforementioned conventional approach which centers elements in space without the scrollbar.

Center horizontally

.horizontal-center {
  position: fixed;
  left: calc((50vw - 50%) * -1); /* add negative value equal to half of the scrollbar width if any */
  transform: translate(calc(50vw - 50%));
}

Center vertically

.vertical-center {
  position: fixed;
  top: calc((50vh - 50%) * -1);
  transform: translate(0, calc(50vh - 50%));
}

Center horizontally and vertically

.center {
  position: fixed;
  left: calc((50vw - 50%) * -1);
  top: calc((50vh - 50%) * -1);
  transform: translate(calc(50vw - 50%), calc(50vh - 50%));
}

A solution using flex box; fully responsive:

parent_div {
    position: fixed;
    width: 100%;
    display: flex;
    justify-content: center;
}

child_div {
    /* whatever you want */
}

You can simply do the following:

div {
  background:pink;
  padding:20px;
  position:fixed;
  
  inset:0;
  margin:auto;
  width: max-content;
  height: max-content;
}

<div>Some content here</div>

if you don't want to use the wrapper method. then you can do this:

.fixed_center_div {
  position: fixed;
  width: 200px;
  height: 200px;
  left: 50%;
  top: 50%;
  margin-left: -100px; /* 50% of width */
  margin-top: -100px; /* 50% of height */
}