What is a clearfix?

If you don't need to support IE9 or lower, you can use flexbox freely, and don't need to use floated layouts.

It's worth noting that today, the use of floated elements for layout is getting more and more discouraged with the use of better alternatives.

display: inline-block - Better

Flexbox - Best (but limited browser support)

Flexbox is supported from Firefox 18, Chrome 21, Opera 12.10, and Internet Explorer 10, Safari 6.1 (including Mobile Safari) and Android's default browser 4.4.

For a detailed browser list see: https://caniuse.com/flexbox.

(Perhaps once its position is established completely, it may be the absolutely recommended way of laying out elements.)

A clearfix is a way for an element to automatically clear its child elements, so that you don't need to add additional markup. It's generally used in float layouts where elements are floated to be stacked horizontally.

The clearfix is a way to combat the zero-height container problem for floated elements

A clearfix is performed as follows:

.clearfix::after {
   content: " "; /* Older browser do not support empty content */
   visibility: hidden;
   display: block;
   height: 0;
   clear: both;
}

Or, if you don't require IE<8 support, the following is fine too:

.clearfix::after {
  content: "";
  display: table;
  clear: both;
}

Normally you would need to do something as follows:

<div>
    <div style="float: left;">Sidebar</div>
    <div style="clear: both;"></div> <!-- Clear the float -->
</div>

With clearfix, you only need the following:

<div class="clearfix">
    <div style="float: left;" class="clearfix">Sidebar</div>
    <!-- No Clearing div! -->
</div>

Read about it in this article - by Chris Coyer @ CSS-Tricks

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The other answers are correct. But I want to add that it is a relic of the time when people were first learning CSS, and abused float to do all their layout. float is meant to do stuff like float images next to long runs of text, but lots of people used it as their primary layout mechanism. Since it wasn't really meant for that, you need hacks like "clearfix" to make it work.

These days display: inline-block is a solid alternative (except for IE6 and IE7), although more modern browsers are coming with even more useful layout mechanisms under names like flexbox, grid layout, etc.

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The clearfix allows a container to wrap its floated children. Without a clearfix or equivalent styling, a container does not wrap around its floated children and collapses, just as if its floated children were positioned absolutely.

There are several versions of the clearfix, with Nicolas Gallagher and Thierry Koblentz as key authors.

If you want support for older browsers, it's best to use this clearfix :

.clearfix:before, .clearfix:after {
    content: "";
    display: table;
}

.clearfix:after {
    clear: both;
}

.clearfix {
    *zoom: 1;
}

In SCSS, you could use the following technique :

%clearfix {
    &:before, &:after {
        content:" ";
        display:table;
    }

    &:after {
        clear:both;
    }

    & {
        *zoom:1;
    }
}

#clearfixedelement {
    @extend %clearfix;
}

If you don't care about supporting older browsers, there's a shorter version :

.clearfix:after {
    content:"";
    display:table;
    clear:both;
}

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To offer an update on the situation on Q2 of 2017.

A new CSS3 display property is available in Firefox 53, Chrome 58 and Opera 45.

.clearfix {
   display: flow-root;
}

Check the availability for any browser here: http://caniuse.com/#feat=flow-root

The element (with a display property set to flow-root) generates a block container box, and lays out its contents using flow layout. It always establishes a new block formatting context for its contents.

Meaning that if you use a parent div containing one or several floating children, this property is going to ensure the parent encloses all of its children. Without any need for a clearfix hack. On any children, nor even a last dummy element (if you were using the clearfix variant with :before on the last children).

.container { display: flow-root; background-color: Gainsboro; } .item { border: 1px solid Black; float: left; } .item1 { height: 120px; width: 120px; } .item2 { height: 80px; width: 140px; float: right; } .item3 { height: 160px; width: 110px; }

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Simply put, clearfix is a hack.

It is one of those ugly things that we all just have to live with as it is really the only reasonable way of ensuring floated child elements don't overflow their parents. There are other layout schemes out there but floating is too commonplace in web design/development today to ignore the value of the clearfix hack.

I personally lean towards the Micro Clearfix solution (Nicolas Gallagher)

.container:before,
.container:after {
  content:"";
  display:table;
}
.container:after {
  clear:both;
}
.container {
  zoom:1; /* For IE 6/7 (trigger hasLayout) */
}

reference

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A technique commonly used in CSS float-based layouts is assigning a handful of CSS properties to an element which you know will contain floating elements. The technique, which is commonly implemented using a class definition called clearfix, (usually) implements the following CSS behaviors:

.clearfix:after {
    content: ".";
    display: block;
    height: 0;
    clear: both;
    visibility: hidden;
    zoom: 1
}

The purpose of these combined behaviors is to create a container :after the active element containing a single '.' marked as hidden which will clear all preexisting floats and effectively reset the the page for the next piece of content.

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I tried out the accepted answer but I still had a problem with the content alignment. Adding a ":before" selector as shown below fixed the issue:

// LESS HELPER
.clearfix()
{
    &:after, &:before{
       content: " "; /* Older browser do not support empty content */
       visibility: hidden;
       display: block;
       height: 0;
       clear: both;
    }
}

LESS above will compile to CSS below:

clearfix:after,
clearfix:before {
  content: " ";
  /* Older browser do not support empty content */
  visibility: hidden;
  display: block;
  height: 0;
  clear: both;
}

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The other (and perhaps simplest) option for acheiving a clearfix is to use overflow:hidden; on the containing element. For example

.parent { background: red; overflow: hidden; } .segment-a { float: left; } .segment-b { float: right; }

Of course this can only be used in instances where you never wish the content to overflow.

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Here is a different method same thing but a little different

the difference is the content dot which is replaced with a \00A0 == whitespace

More on this http://www.jqui.net/tips-tricks/css-clearfix/

.clearfix:after { content: "\00A0"; display: block; clear: both; visibility: hidden; line-height: 0; height: 0;}
.clearfix{ display: inline-block;}
html[xmlns] .clearfix { display: block;}
* html .clearfix{ height: 1%;}
.clearfix {display: block}

Here is a compact version of it...

.clearfix:after { content: "\00A0"; display: block; clear: both; visibility: hidden; line-height: 0; height: 0;width:0;font-size: 0px}.clearfix{ display: inline-block;}html[xmlns] .clearfix { display: block;}* html .clearfix{ height: 1%;}.clearfix {display: block}

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