Css – What, exactly, is needed for “margin: 0 auto;” to work

css

I know that setting margin: 0 auto; on an element is used to centre it (left-right). However, I know that the element and its parent must meet certain criteria for the auto margin to work, and I can never seem to get the magic right.

So my question is simple: what CSS properties have to be set on an element and its parent in order for margin: 0 auto; to left-right centre the child?

Best Answer

Off the top of my head:

The element must be block-level, e.g. display: block or display: table
The element must not float
The element must not have a fixed or absolute position¹

Off the top of other people's heads:

The element must have a width that is not auto²

Note that all of these conditions must be true of the element being centered for it to work.

¹ _{There is one exception to this: if your fixed or absolutely positioned element has left: 0; right: 0, it will center with auto margins.}

² _{Technically, margin: 0 auto does work with an auto width, but the auto width takes precedence over the auto margins, and the auto margins are zeroed out as a result, making it seem as though they "don't work".}

Related Solutions

CSS 100% height with padding/margin

I learned how to do these sort of things reading "PRO HTML and CSS Design Patterns". The display:block is the default display value for the div, but I like to make it explicit. The container has to be the right type; position attribute is fixed, relative, or absolute.

.stretchedToMargin {
  display: block;
  position:absolute;
  height:auto;
  bottom:0;
  top:0;
  left:0;
  right:0;
  margin-top:20px;
  margin-bottom:20px;
  margin-right:80px;
  margin-left:80px;
  background-color: green;
}

<div class="stretchedToMargin">
  Hello, world
</div>

Fiddle by Nooshu's comment

Html – re way to make a child DIV’s width wider than the parent DIV using CSS

Here's a generic solution that keeps the child element in the document flow:

.child {
  width: 100vw;
  position: relative;
  left: calc(-50vw + 50%);
}

We set the width of the child element to fill the entire viewport width, then we make it meet the edge of the screen by moving it to the left by a distance of half the viewport, minus 50% of the parent element's width.

Demo:

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  overflow-x: hidden;
}

.parent {
  max-width: 400px;
  margin: 0 auto;
  padding: 1rem;
  position: relative;
  background-color: darkgrey;
}

.child {
  width: 100vw;
  position: relative;
  left: calc(-50vw + 50%);

  height: 100px;
  border: 3px solid red;
  background-color: lightgrey;
}

<div class="parent">
  Pre
  <div class="child">Child</div>
  Post
</div>

Browser support for vw and for calc() can generally be seen as IE9 and newer.

Note: This assumes the box model is set to border-box. Without border-box, you would also have to subtract paddings and borders, making this solution a mess.

Note: It is encouraged to hide horizontal overflow of your scrolling container, as certain browsers may choose to display a horizontal scrollbar despite there being no overflow.

Best Answer

Related Solutions

CSS 100% height with padding/margin

Html – re way to make a child DIV’s width wider than the parent DIV using CSS

Here's a generic solution that keeps the child element in the document flow:

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