flex

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:

<div class="flex-container">
  <div class="item auto">auto</div>
  <div class="item auto">auto</div>
  <div class="item auto">auto</div>
</div>

<div class="flex-container">
  <div class="item auto">auto</div>
  <div class="item initial">initial</div>
  <div class="item initial">initial</div>
</div>

<div class="flex-container">
  <div class="item auto">auto</div>
  <div class="item auto">auto</div>
  <div class="item none">none</div>
</div>

<div class="flex-container">
  <div class="item initial">initial</div>
  <div class="item none">none</div>
  <div class="item none">none</div>
</div>

<div class="flex-container">
  <div class="item four">4</div>
  <div class="item two">2</div>
  <div class="item one">1</div>
</div>

* {
  box-sizing: border-box;
}

.flex-container {
  background-color: #f4f7f8;
  resize: horizontal;
  overflow: hidden;
  display: flex;
  margin: 1em;
}

.item {
  margin: 1em;
  padding: 0.5em;
  width: 110px;
  min-width: 0;
  background-color: #1b5385;
  color: white;
  font-family: monospace;
  font-size: 13px;
}

.initial {
  flex: initial;
}

.auto {
  flex: auto;
}

.none {
  flex: none;
}

.four {
  flex: 4;
}

.two {
  flex: 2;
}

.one {
  flex: 1;
}

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'> ]  

HTML

<div id="flex-container">
  <div class="flex-item" id="flex">Flex box (click to toggle raw box)</div>
  <div class="raw-item" id="raw">Raw box</div>
</div>

CSS

#flex-container {
  display: flex;
  flex-direction: row;
}

#flex-container > .flex-item {
  flex: auto;
}

#flex-container > .raw-item {
  width: 5rem;
}

const flex = document.getElementById("flex");
const raw = document.getElementById("raw");
flex.addEventListener("click", () => {
  raw.style.display = raw.style.display === "none" ? "block" : "none";
});

#flex-container {
  width: 100%;
  font-family: Consolas, Arial, sans-serif;
}

#flex-container > div {
  border: 1px solid #f00;
  padding: 1rem;
}

#flex-container > .raw-item {
  border: 1px solid #000;
}

Result

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