This property is a shorthand for the following CSS properties:

• flex-grow
• flex-shrink
• flex-basis

/* Keyword values */
flex: auto;
flex: initial;
flex: none;

/* One value, unitless number: flex-grow
flex-basis is then equal to 0. */
flex: 2;

/* One value, width/height: flex-basis */
flex: 10em;
flex: 30%;
flex: min-content;

/* Two values: flex-grow | flex-basis */
flex: 1 30px;

/* Two values: flex-grow | flex-shrink */
flex: 2 2;

/* Three values: flex-grow | flex-shrink | flex-basis */
flex: 2 2 10%;

/* Global values */
flex: inherit;
flex: initial;
flex: revert;
flex: revert-layer;
flex: unset;

The flex property may be specified using one, two, or three values.

• One-value syntax: the value must be one of:
  • a valid value for <flex-grow>: then the shorthand expands to flex: <flex-grow> 1 0.
  • a valid value for <flex-basis>: then the shorthand expands to flex: 1 1 <flex-basis>.
  • the keyword none or one of the global keywords.
• Two-value syntax:
  • The first value must be a valid value for flex-grow.
  • The second value must be one of:
    • a valid value for flex-shrink: then the shorthand expands to flex: <flex-grow> <flex-shrink> 0.
    • a valid value for flex-basis: then the shorthand expands to flex: <flex-grow> 1 <flex-basis>.
• Three-value syntax: the values must be in the following order:
  1. a valid value for flex-grow.
  2. a valid value for flex-shrink.
  3. a valid value for flex-basis.

initial

The item is sized according to its width and height properties. It shrinks to its minimum size to fit the container, but does not grow to absorb any extra free space in the flex container. This is equivalent to setting "flex: 0 1 auto".

auto

The item is sized according to its width and height properties, but grows to absorb any extra free space in the flex container, and shrinks to its minimum size to fit the container. This is equivalent to setting "flex: 1 1 auto".

none

The item is sized according to its width and height properties. It is fully inflexible: it neither shrinks nor grows in relation to the flex container. This is equivalent to setting "flex: 0 0 auto".

<'flex-grow'>

Defines the flex-grow of the flex item. Negative values are considered invalid. Defaults to 1 when omitted. (initial is 0)

<'flex-shrink'>

Defines the flex-shrink of the flex item. Negative values are considered invalid. Defaults to 1 when omitted. (initial is 1)

<'flex-basis'>

Defines the flex-basis of the flex item. A preferred size of 0 must have a unit to avoid being interpreted as a flexibility. Defaults to 0 when omitted. (initial is auto)

For most purposes, authors should set flex to one of the following values: auto, initial, none, or a positive unitless number. To see the effect of these values, try resizing the flex containers below:

By default flex items don't shrink below their minimum content size. To change this, set the item's min-width or min-height.

flex = 
  none                                                |
  [ <'flex-grow'> <'flex-shrink'>? || <'flex-basis'> ]  

This example shows how a flex item with flex: auto grows to absorb any free space in the container.

HTML

<div id="flex-container">
  <div id="flex-auto">flex: auto (click to toggle raw box)</div>
  <div id="flex-initial">flex: initial</div>
</div>

CSS

#flex-container {
  display: flex;
  font-family: Consolas, Arial, sans-serif;
}

#flex-container > div {
  padding: 1rem;
}

#flex-auto {
  flex: auto;
  border: 1px solid #f00;
}

#flex-initial {
  border: 1px solid #000;
}

JavaScript

const flexAuto = document.getElementById("flex-auto");
const flexInitial = document.getElementById("flex-initial");
flexAuto.addEventListener("click", () => {
  flexInitial.style.display =
    flexInitial.style.display === "none" ? "block" : "none";
});

Result

The flex container contains two flex items:

• "flex: auto" has a flex value of auto
• "flex: initial" has a flex value of initial

The "flex: initial" item takes up as much space as its width requires, but does not expand to take up any more space. All the remaining space is taken up by "flex: auto".

When you click "flex: auto", we set "flex: initial"'s display property to none, removing it from the layout. The "flex: auto" item then expands to occupy all the available space in the container.

• CSS Flexbox Guide: Basic Concepts of Flexbox
• CSS Flexbox Guide: Controlling Ratios of flex items along the main axis