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---
title: flex-basis
slug: Web/CSS/flex-basis
translation_of: Web/CSS/flex-basis
---
<div>{{CSSRef}}</div>
<p>The <strong><code>flex-basis</code></strong> <a href="/en-US/docs/Web/CSS">CSS</a> property sets the initial main size of a flex item. It sets the size of the content box unless otherwise set with {{Cssxref("box-sizing")}}.</p>
<div>{{EmbedInteractiveExample("pages/css/flex-basis.html")}}</div>
<div class="note">
<p><strong>Note:</strong> in case both <code>flex-basis</code> (other than <code>auto</code>) and <code>width</code> (or <code>height</code> in case of <code>flex-direction: column</code>) are set for an element, <code>flex-basis</code> has priority.</p>
</div>
<h2 id="Syntax">Syntax</h2>
<pre class="brush:css no-line-numbers">/* Specify <'width'> */
flex-basis: 10em;
flex-basis: 3px;
flex-basis: auto;
/* Intrinsic sizing keywords */
flex-basis: fill;
flex-basis: max-content;
flex-basis: min-content;
flex-basis: fit-content;
/* Automatically size based on the flex item’s content */
flex-basis: content;
/* Global values */
flex-basis: inherit;
flex-basis: initial;
flex-basis: unset;
</pre>
<p>The <code>flex-basis</code> property is specified as either the keyword <code><a href="#content">content</a></code> or a <code><a href="#<'width'>"><'width'></a></code>.</p>
<h3 id="Values">Values</h3>
<dl>
<dt><a id="<'width'>" name="<'width'>"><code><'width'></code></a></dt>
<dd>An absolute {{cssxref("<length>")}}, a {{cssxref("<percentage>")}} of the parent flex container's main size property, or the keyword <code>auto</code>. Negative values are invalid. Defaults to <code>auto</code>.</dd>
<dt><a id="content" name="content"><code>content</code></a></dt>
<dd>Indicates automatic sizing, based on the flex item’s content.</dd>
<dd>
<div class="note"><strong>Note:</strong> This value was not present in the initial release of Flexible Box Layout, and thus some older implementations will not support it. The equivalent effect can be had by using <code>auto</code> together with a main size (<a href="https://drafts.csswg.org/css2/visudet.html#propdef-width">width</a> or <a href="https://drafts.csswg.org/css2/visudet.html#propdef-height">height</a>) of <code>auto</code>.</div>
<div class="note">
<p id="comment_text_0"><strong>History:</strong></p>
<ul>
<li>Originally, <code>flex-basis:auto</code> meant "look at my <code>width</code> or <code>height</code> property".</li>
<li>Then, <code>flex-basis:auto</code> was changed to mean automatic sizing, and "main-size" was introduced as the "look at my <code>width</code> or <code>height</code> property" keyword. It was implemented in <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=1032922" title='RESOLVED FIXED - Rename "flex-basis:auto" to "main-size", while preserving "flex:auto" shorthand value'>bug 1032922</a>.</li>
<li>Then, that change was reverted in <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=1093316" title='RESOLVED FIXED - Back out flexbox "flex-basis:main-size" rename, since the CSSWG removed it from the spec'>bug 1093316</a>, so <code>auto</code> once again means "look at my <code>width</code> or <code>height</code> property"; and a new <code>content</code> keyword is being introduced to trigger automatic sizing. ({{bug("1105111")}} covers adding that keyword).</li>
</ul>
</div>
</dd>
</dl>
<h3 id="Formal_syntax">Formal syntax</h3>
{{csssyntax}}
<h2 id="Example">Example</h2>
<h3 id="HTML">HTML</h3>
<pre class="brush: html"><ul class="container">
<li class="flex flex1">1: flex-basis test</li>
<li class="flex flex2">2: flex-basis test</li>
<li class="flex flex3">3: flex-basis test</li>
<li class="flex flex4">4: flex-basis test</li>
<li class="flex flex5">5: flex-basis test</li>
</ul>
<ul class="container">
<li class="flex flex6">6: flex-basis test</li>
</ul>
</pre>
<h3 id="CSS">CSS</h3>
<pre class="brush: css">.container {
font-family: arial, sans-serif;
margin: 0;
padding: 0;
list-style-type: none;
display: flex;
flex-wrap: wrap;
}
.flex {
background: #6AB6D8;
padding: 10px;
margin-bottom: 50px;
border: 3px solid #2E86BB;
color: white;
font-size: 20px;
text-align: center;
position: relative;
}
.flex:after {
position: absolute;
z-index: 1;
left: 0;
top: 100%;
margin-top: 10px;
width: 100%;
color: #333;
font-size: 18px;
}
.flex1 {
flex-basis: auto;
}
.flex1:after {
content: 'auto';
}
.flex2 {
flex-basis: max-content;
}
.flex2:after {
content: 'max-content';
}
.flex3 {
flex-basis: min-content;
}
.flex3:after {
content: 'min-content';
}
.flex4 {
flex-basis: fit-content;
}
.flex4:after {
content: 'fit-content';
}
.flex5 {
flex-basis: content;
}
.flex5:after {
content: 'content';
}
.flex6 {
flex-basis: fill;
}
.flex6:after {
content: 'fill';
}
</pre>
<h3 id="Results">Results</h3>
<p>{{EmbedLiveSample('Example', '860', '360', '', 'Web/CSS/flex-basis')}}</p>
<h2 id="Specifications">Specifications</h2>
<table class="standard-table">
<thead>
<tr>
<th>Specification</th>
<th>Status</th>
<th>Comment</th>
</tr>
</thead>
<tbody>
<tr>
<td>{{SpecName('CSS3 Flexbox', '#propdef-flex-basis', 'flex-basis')}}</td>
<td>{{Spec2('CSS3 Flexbox')}}</td>
<td>Initial definition</td>
</tr>
</tbody>
</table>
<p>{{cssinfo}}</p>
<h2 id="Browser_compatibility">Browser compatibility</h2>
<p>{{Compat("css.properties.flex-basis")}}</p>
<h2 id="See_also">See also</h2>
<ul>
<li>CSS Flexbox Guide: <em><a href="/en-US/docs/Web/CSS/CSS_Flexible_Box_Layout/Basic_Concepts_of_Flexbox">Basic Concepts of Flexbox</a></em></li>
<li>CSS Flexbox Guide: <em><a href="/en-US/docs/Web/CSS/CSS_Flexible_Box_Layout/Controlling_Ratios_of_Flex_Items_Along_the_Main_Ax">Controlling Ratios of flex items along the main axis</a></em></li>
<li>{{cssxref("width")}}</li>
</ul>
<div id="eJOY__extension_root" style=""></div>
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