/* <number> values */
flex-grow: 3;
flex-grow: 0.6;

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

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

<number>

See <number>. Negative values are invalid. Defaults to 0.

This property specifies how much of the remaining space in the flex container should be assigned to the item (the flex grow factor).

The main size is either width or height of the item which is dependent on the flex-direction value.

The remaining space is the size of the flex container minus the size of all flex items' sizes together. If all sibling items have the same flex grow factor, then all items will receive the same share of remaining space, otherwise it is distributed according to the ratio defined by the different flex grow factors.

flex-grow is used alongside the other flex properties flex-shrink and flex-basis, and normally defined using the flex shorthand to ensure all values are set.

flex-grow = 
  <number [0,∞]>  

In this example, there is a total of 8 growth factors distributed among the 6 flex items, meaning each growth factor is 12.5% of the remaining space.

HTML

<h4>This is a Flex-Grow</h4>
<h5>A,B,C and F are flex-grow:1 . D and E are flex-grow:2 .</h5>
<div id="content">
  <div class="small" style="background-color:red;">A</div>
  <div class="small" style="background-color:lightblue;">B</div>
  <div class="small" style="background-color:yellow;">C</div>
  <div class="double" style="background-color:brown;">D</div>
  <div class="double" style="background-color:lightgreen;">E</div>
  <div class="small" style="background-color:brown;">F</div>
</div>

CSS

#content {
  display: flex;

  justify-content: space-around;
  flex-flow: row wrap;
  align-items: stretch;
}

.small {
  flex-grow: 1;
  border: 3px solid rgba(0, 0, 0, 0.2);
}

.double {
  flex-grow: 2;
  border: 3px solid rgba(0, 0, 0, 0.2);
}

Result

When the six flex items are distributed along the container's main axis, if the sum of the main content of those flex items is less than the size of the container's main axis, the extra space is distributed among the size flex items, with A, B, C, and F, each getting 12.5% of the remaining space and D and E each getting 25% of the extra space.

• CSS Flexbox Guide: Basic Concepts of Flexbox
• CSS Flexbox Guide: Controlling Ratios of flex items along the main axis
• flex-grow is weird. Or is it? article by Manuel Matuzovic on CSS-Tricks, which illustrates how flex-grow works