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Warning: This is a property for controlling parts of the XUL box model. It does not match either the old CSS Flexible Box Layout Module drafts for 'box-flex' (which were based on this property) or the behavior of '-webkit-box-flex' (which is based on those drafts).

See Flexbox for more information on what you should be using instead of this property.

The -moz-box-flex and -webkit-box-flex CSS properties specify how a -moz-box or -webkit-box grows to fill the box that contains it, in the direction of the containing box's layout. See Flexbox for more about the properties of flexbox elements.

/* <number> values */
-moz-box-flex: 0;
-moz-box-flex: 2;
-moz-box-flex: 3.5;
-webkit-box-flex: 0;
-webkit-box-flex: 2;
-webkit-box-flex: 3.5;

/* Global values */
-moz-box-flex: inherit;
-moz-box-flex: initial;
-moz-box-flex: unset;
-webkit-box-flex: inherit;
-webkit-box-flex: initial;
-webkit-box-flex: unset;

Initial value0
Applies toelements that are direct children of an element with a CSS display value of -moz-box or -moz-inline-box or -webkit-box or -webkit-inline-box
Computed valueas specified
Animation typediscrete
Canonical orderthe unique non-ambiguous order defined by the formal grammar



If the value is 0, the box does not grow. If it is greater than 0, the box grows to fill a proportion of the available space.

Formal syntax



<!DOCTYPE html>
    <title>-moz-box-flex example</title>
      div.example {
        display: -moz-box;
        display: -webkit-box;
        border: 1px solid black;
        width: 100%;                
      div.example > p:nth-child(1) {
        -moz-box-flex: 1;       /* Mozilla */
        -webkit-box-flex: 1;    /* WebKit */
        border: 1px solid black;
      div.example > p:nth-child(2) {
        -moz-box-flex: 0;       /* Mozilla */
        -webkit-box-flex: 0;    /* WebKit */
        border: 1px solid black;
    <div class="example">
      <p>I will expand to fill extra space</p>
      <p>I will not expand</p>


The containing box allocates the available extra space in proportion to the flex value of each of the content elements.

Content elements that have zero flex do not grow.

If only one content element has nonzero flex, then it grows to fill the available space.

Content elements that have the same flex grow by the same absolute amounts.

If the flex value is set using the element's flex attribute, then the style is ignored.

To make XUL elements in a containing box the same size, set the containing box's equalsize attribute to the value always. This attribute does not have a corresponding CSS property.

A trick to make all content elements in a containing box the same size, is to give them all a fixed size (e.g. height: 0), and the same box-flex value greater than zero (e.g. -moz-box-flex: 1).


This property is a non-standard extension. There was an old draft of the CSS3 Flexbox specification that defined a box-flex property, but that draft has since been superseded.

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Yes)-webkit (Yes)-webkit (Yes)-moz[1] No support (Yes)-webkit 1.1-khtml
Feature Android Chrome for Android Edge Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support ? ? (Yes)-webkit ? No support ? 1.0-webkit

[1] In addition to the -moz prefixed support, Gecko 48.0 (Firefox 48.0 / Thunderbird 48.0 / SeaMonkey 2.45) added support for a -webkit prefixed version of the property for web compatibility reasons behind the preference layout.css.prefixes.webkit, defaulting to false. Since Gecko 49.0 (Firefox 49.0 / Thunderbird 49.0 / SeaMonkey 2.46) the preference defaults to true.

See also

box-orient, box-pack, box-direction, flex