Dan D Kim

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CSS Flex Ramp Up

2020-12-21 Dan D. Kimcss

Ever wanted to flex off your CSS skills but never got around learning it properly?

Well then, this guide is for you to flex 💪

The learnings in this post are coming from Rachel Andrew's Codepen

Table of Contents

Playground Setup

You can skip the setup and go straight to the next section if you just wanna read along.

Let’s add some elements to our HTML.

<div class="container">
    <div class="item">One</div>
    <div class="item">Two</div>
    <div class="item">Three</div>
    <div class="item">Four</div>
    <div class="item">Five</div>

And put in some CSS to make them look bearable.

* { box-sizing: border-box; }

body {
    font: 1.1em/1.3 Tahoma, Verdana, Segoe, sans-serif;
    background-color: #fff;
    color: #333;
    max-width: 760px;
    margin: 1em auto;

.item {
    padding: 10px;
    border: 5px solid rgb(17, 105, 121);
    background-color: rgba(17, 105, 121,.1);
    border-radius: 5px;

No need to understand those, as they aren’t really what this post is about. We are just making them look like this


Let’s set up our container with display set to flex.

.contaier {
  display: flex;

Nothing really changed in our UI.

CSS Flex guide


Let’s make the elements appear in a column inside of a row.

.container {
  display: flex;
  flex-direction: column;

Now we get a column.

CSS Flex guide


Instead of making it entirely a row or a column, let’s try to make the elements wrap around dynamically.

.container {
  display: flex;
  border: 2px solid black;
  flex-direction: row;
  flex-wrap: wrap;}

This will allow our elements to be wrapped.

CSS Flex Guide

flex: <‘flex-grow’> <‘flex-shrink’> <‘flex-basis’>

Now, let’s apply flex to our item class.

You might want to play around with this part in the codepen to get the hang of it.

.item {
  flex: 0 1 150px;

We are basically doing the following:

  • flex-grow is 0, meaning the elements will not expand to fill up empty space.
  • flex-shrink is 1, meaning all the items will shrink appropriately as the width of the parent gets smaller
  • flex-basis is set to 150px, meaning the element should start at 150px before space is evenly distributed among the elements.

flex-grow and flex-shrink are relative properties, meaning elements with a higher value will expand more relative to other elements.

So we will get the following:

CSS Flex Guide

Also notice this:

CSS Flex Guide

Let’s set 1 to flex-grow.

.item {
  flex: 1 1 150px;

Then we will get the following:

CSS Flex Guide

To become a master at flexing, I recommend checking out the docs and playing around on the Codepen (fork mine or something).


Few notes about flex.

  • one-value syntax like flex: <num> will result in flex: <num> 1 0;
  • avoid two-value syntax cuz it’s confusing
  • there are also other shorthand values like initial, auto, none
    • initial is equal to 0 1 auto
    • auto is equal to 1 1 auto
    • none is equal to 0 0 auto
  • usually, the shorthand or the one-value syntax is more commonly used instead of the two and three-value syntax


You get a huge variety of alignment options with flex. Instead of writing about it, I just wanted to guide you to this useful doc. Check it out and play around on Codepen. Here is one from Rachel Andrew.

Quick Notes

  • justify-content
    • align content along the main axis (flex-direction)
  • align-items
    • align content along the cross-axis (perpendicular to flex-direction)
  • align-content
    • manage how content aligns when there is extra space in the parent container. Aligns along the cross-axis.
  • align-self
    • override the cross-axis alignment on individual elements

Relative properties

In the flex section I mentioned “relative properties”. This means that elements will take up space relative to other elements based on their grow / shrink values.

Let’s say that we want to give our first div a lot more growth space than the others.

We can acheive this by giving our first element a higher value of flex-grow than the other items.

.container :nth-child(1) {
  flex: 3 /* 3 1 auto */

.item {
  flex: auto; /* 1 1 auto*/

CSS Flex Guide

CSS Flex Guide

That’s all. Hope you learned something. Happy coding!