CSS Flexbox

Dr. Greg Bernstein

Updated September 14th, 2021

CSS Layout Flexbox

Learning Objectives

  • Understand flexbox as a layout technique based around a primary (single) axis
  • Understand properties that apply to the flex container versus the flex items
  • Understand that you have control over main axis, cross axis properties, and wrapping related properties

Readings and More

  1. MDN Flexbox

  2. A Complete Guide to Flexbox

  3. Flexbox Froggy “a game where you help Froggy and friends by writing CSS code!”

Live Practice Files

Experiment with these are you read these notes:

  • From MDN flexbox0.html, container is the section element

  • From me: physics.html, container is the p element holding the links to other pages.

  • From me: VerticalTides.html container is the body element, uses main axis, cross axis, and item properties

What is it?

From MDN:

Flexbox is a one-dimensional layout method for laying out items in rows or columns. Items flex to fill additional space and shrink to fit into smaller spaces.

What is it? II

From CSS Tricks:

The main idea behind the flex layout is to give the container the ability to alter its items’ width/height (and order) to best fill the available space (mostly to accommodate to all kind of display devices and screen sizes). A flex container expands items to fill available free space, or shrinks them to prevent overflow.

Why Flexbox

From MDN: “The following simple layout requirements are either difficult or impossible to achieve with such tools, in any kind of convenient, flexible way:”

  • Vertically centering a block of content inside its parent.
  • Making all the children of a container take up an equal amount of the available width/height, regardless of how much width/height is available.
  • Making all columns in a multiple column layout adopt the same height even if they contain a different amount of content.

Simple Flexbox Example I

Example from MDN, three articles within a section, default (no flex):

Page with no flexbox applied

Simple Flexbox Example II

Applying display: flex to the section containing the articles:

Page with flexbox applied

Basic Model/Terminology I

From MDN:

  • The main axis is the axis running in the direction the flex items are being laid out in. The start and end of this axis are called the main start and main end.
  • The cross axis is the axis running perpendicular to the direction the flex items are being laid out in. The start and end of this axis are called the cross start and cross end.

Basic Model/Terminology II

From MDN:

  • The parent element that has display: flex set on it is called the flex container.
  • The items being laid out as flexible boxes inside the flex container are called flex items.

Basic Model/Terminology III

From MDN:

Flex terms diagram

Flex Container Properties

display and flex-direction

  • Use display: flex; on the container element to enable flex layout.
  • Use flex-direction: property, where property = row, row-reverse, column or column-reverse
  • The default flex-direction is row


From CSS-Tricks:

This defines the alignment along the main axis. It helps distribute extra free space left over when either all the flex items on a line are inflexible, or are flexible but have reached their maximum size.

justify-content values I

From CSS-Tricks:

  • flex-start (default): items are packed toward the start line
  • flex-end: items are packed toward to end line
  • center: items are centered along the line
  • space-between: items are evenly distributed in the line; first item is on the start line, last item on the end line

justify-content values II

From CSS-Tricks:

  • space-around: items are evenly distributed in the line with equal space around them.
  • space-evenly: items are distributed so that the spacing between any two items (and the space to the edges) is equal.

justify-content example I


Justify content: space around

justify-content example II


Justify content: space between

justify-content example III


Justify content: center


From CSS-Tricks:

This defines the default behavior for how flex items are laid out along the cross axis on the current line.

align-items values

From CSS-Tricks:

  • start: cross-start margin edge of the items is placed on the cross-start line
  • end: cross-end margin edge of the items is placed on the cross-end line
  • center: items are centered in the cross-axis
  • baseline: items are aligned such as their baselines align
  • stretch (default): stretch to fill the container (still respect min-width/max-width)

align-items: start

Align items start demo

align-items: end

Align items end demo

align-items: center

Align items center demo

flex-wrap and flex-flow

From CSS-Tricks:

  • By default, flex items will all try to fit onto one line. You can change that and allow the items to wrap as needed with the flex-wrap property.
  • flex-wrap values: nowrap, wrap, wrap-reverse
  • flex-flow is a shorthand flex-direction and flex-wrap properties, which together define the flex container’s main and cross axes.
  • Default is row nowrap.


From CSS-Tricks align-content:

This aligns a flex container’s lines within when there is extra space in the cross-axis, similar to how justify-content aligns individual items within the main-axis.

Note: this property has no effect when there is only one line of flex items.

Flex Item Properties

order and flex-grow

From CSS-Tricks:

  • order: “By default, flex items are laid out in the source order. However, the order property controls the order in which they appear in the flex container.”

  • flex-grow: “This defines the ability for a flex item to grow if necessary. It accepts a unitless value that serves as a proportion. It dictates what amount of the available space inside the flex container the item should take up.”

flex-shrink and flex-basis

From CSS-Tricks:

  • flex-shrink: This defines the ability for a flex item to shrink if necessary.
  • flex-basis: This defines the default size of an element before the remaining space is distributed. It can be a length (e.g. 20%, 5rem, etc.) or a keyword.


From CSS-Tricks:

This allows the default alignment (or the one specified by align-items) to be overridden for individual flex items.

Note: The default alignment is the alignment set on the container.

Vertical Layout

Vertical Layout Example

  • Centering elements horizontal (some or all)
  • Set <nav> near top and <footer> at bottom with extra space in between
  • Vertical Spacing tip: For this to work you must set the height property on the flex container


See file VerticalTides.html

<!DOCTYPE html>
<html lang="en">
    <meta charset="utf-8">
    <title>Berkeley Tides</title>
            <li><a href="index.html">Home</a></li>
            <li><a href="about.html">About</a></li>
            <li><a href="activities.html">Activities</a></li>
            <li><a href="membership.html">Membership</a></li>
            <li class="active"><a href="tides.html">Tides</a></li>
            <li><a href="login.html">Login</a></li>
            <h1>Berkeley Tides</h1>
        <p>Information about tides in the Berkeley South sailing basin. To be furnished...</p>
        <p>🐳 &copy; 2020 Grotto Networking! 🐬</p>

No Styling

No flexbox applied

Flex for Centering

body {flex-direction: column; align-items: center;}

Center all vertical blocks

Vertical Spacing

body {/* previous stuff */ justify-content: space-between; height: 92vh;}

Vertical spacing demo

Adjust nav to Left

nav {align-self: start;}

Align self demo

Final CSS

See file VerticalTides.css

/* Only those parts relevent to the flexbox demo */
body {
    margin: 1em;
    justify-content: space-between; /* Vertical space between */
    height: 92vh; /* Set height for vertical spacing  */

nav {
    align-self:start; /* over ride the containers "centering" */
    background-color: white;
    max-width: 10em;
    padding: 0.1em;
    border:aqua solid;
    border-radius: 1em;
// reveal.js plugins