[html] Flexbox: center horizontally and vertically

How to Center Elements Vertically and Horizontally in Flexbox

Below are two general centering solutions.

One for vertically-aligned flex items (flex-direction: column) and the other for horizontally-aligned flex items (flex-direction: row).

In both cases the height of the centered divs can be variable, undefined, unknown, whatever. The height of the centered divs doesn't matter.

Here's the HTML for both:

<div id="container"><!-- flex container -->

    <div class="box" id="bluebox"><!-- flex item -->
        <p>DIV #1</p>
    </div>

    <div class="box" id="redbox"><!-- flex item -->
        <p>DIV #2</p>
    </div>

</div>

CSS (excluding decorative styles)

When flex items are stacked vertically:

#container {
    display: flex;           /* establish flex container */
    flex-direction: column;  /* make main axis vertical */
    justify-content: center; /* center items vertically, in this case */
    align-items: center;     /* center items horizontally, in this case */
    height: 300px;
}

.box {
    width: 300px;
    margin: 5px;
    text-align: center;     /* will center text in <p>, which is not a flex item */
}

DEMO

When flex items are stacked horizontally:

Adjust the flex-direction rule from the code above.

#container {
    display: flex;
    flex-direction: row;     /* make main axis horizontal (default setting) */
    justify-content: center; /* center items horizontally, in this case */
    align-items: center;     /* center items vertically, in this case */
    height: 300px;
}

DEMO

Centering the content of the flex items

The scope of a flex formatting context is limited to a parent-child relationship. Descendants of a flex container beyond the children do not participate in flex layout and will ignore flex properties. Essentially, flex properties are not inheritable beyond the children.

Hence, you will always need to apply display: flex or display: inline-flex to a parent element in order to apply flex properties to the child.

In order to vertically and/or horizontally center text or other content contained in a flex item, make the item a (nested) flex container, and repeat the centering rules.

.box {
    display: flex;
    justify-content: center;
    align-items: center;        /* for single line flex container */
    align-content: center;      /* for multi-line flex container */
}

More details here: How to vertically align text inside a flexbox?

Alternatively, you can apply margin: auto to the content element of the flex item.

p { margin: auto; }

Learn about flex auto margins here: Methods for Aligning Flex Items (see box#56).

Centering multiple lines of flex items

When a flex container has multiple lines (due to wrapping) the align-content property will be necessary for cross-axis alignment.

From the spec:

8.4. Packing Flex Lines: the align-content property

The align-content property aligns a flex container’s lines within the flex container when there is extra space in the cross-axis, similar to how justify-content aligns individual items within the main-axis. Note, this property has no effect on a single-line flex container.

More details here: How does flex-wrap work with align-self, align-items and align-content?

Browser support

Flexbox is supported by all major browsers, except IE < 10. Some recent browser versions, such as Safari 8 and IE10, require vendor prefixes. For a quick way to add prefixes use Autoprefixer. More details in this answer.

Centering solution for older browsers

For an alternative centering solution using CSS table and positioning properties see this answer: https://stackoverflow.com/a/31977476/3597276