Non-standard: This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.

Deprecated: This feature is no longer recommended. Though some browsers might still support it, it may have already been removed from the relevant web standards, may be in the process of being dropped, or may only be kept for compatibility purposes. Avoid using it, and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.

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 information about the current standard.

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.


/* <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: revert;
-moz-box-flex: revert-layer;
-moz-box-flex: unset;
-webkit-box-flex: inherit;
-webkit-box-flex: initial;
-webkit-box-flex: revert;
-webkit-box-flex: revert-layer;
-webkit-box-flex: unset;

The box-flex property is specified as a <number>. 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.


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).

Formal definition

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

Formal syntax

box-flex =


Setting box-flex

<!doctype html>
<html lang="en-US">
    <meta charset="UTF-8" />
    <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>


Not part of any standard.

Browser compatibility

BCD tables only load in the browser

See also