Getting started with CSS Flexbox


  • The widths and heights of flexboxes vary to adapt to the display space
  • The display order of flex boxes is independant of their order in the source code
  • Margins of adjacent flex items do not collapse
  • Main axis = vertical = columns
  • Cross axis = horizontal = rows
  • All flex items on a line are equal height by default (the default for axis properties like justify-content, align-content and align-items etc. is stretch), and take the width of the content inside
  • All flex items stay on the same line by default until unless you define flex-wrap: wrap. Without it, it’ll overflow, but not wrap

Why Flexbox

Why not percentages? Percentages are explicit dimensions, you’re telling the browser one fix percentage. The button is always going to be 10%, no matter how big your TV is. Flexbox sizes elements by the size of their content. So the button will be the size of it’s text. You can decide an initial size and then you can decide whether it should grow (flex: auto) or shrink (flex: none) - Flexbox is great for aligning stuff. When you align items with floats, they wrap with their respective floats, what was floated to the right will wrap and still stay on the right of the next line, with all this empty space on it’s left. It looks a bit weird. Consider meta info section of an article, you wan’t everything right aligned on smaller screens but split on the same line when there is enough space. If you achieve the splitting part with floats, you get the wonky wrap issue. Flex is perfect here. flex-wrap wraps in the same direction of flex-direction. example - You can add flex styles to existing layouts and they’ll work. Flex will just take over and the old ones will become fallback

<-> justify-content
<-> flex-direction
^ align-items
^ align-self


flex-flow: flex-direction flex-wrap

flex: flex-grow flex-shrink flex-basis

Make an element a Flex container

.container {
  display: flex; /* block-level container */
  display: inline-flex; /* inline-level container */

Every element inside a flex container automatically becomes a flex item.


You determine direction with either the order or the flex-direction properties. Changing the direction moves the flex items independent of their HTML source order.

flex-direction: row (default) / row-reverse / column / column-reverse
order: 0 (default)
  • Your main axis changes when you change direction. This is important to keep in mind when you are confused it didn’t apply justif-content or align-content, you probably have the direction changed and the main axis has changed as a result
  • order can also take negative values, e.g. -3
  • changing the order/direction only changes the items visually. If you try to copy or highlight it (e.g. some text), it’ll do it in the order the HTML source is, regardless of what order you have it showing in your flex layout


flex determines the dimensions.

flex: flex-grow flex-shrink flex-basis
flex: 1; /* all flex items will take one equal part */

If there isn’t enough space, the size will be determined by the content inside.

  • flex-grow = how much flex items will grow relative to other items if extra space is available (proportion of extra space that it gets)
  • flex-shrink = how much item will shrink relative to others if there is not enough space (proportion of overflow that gets shaved off)
  • flex-basis = the initial starting point before space is distributed (any standard width/height value including auto)
input {
  flex: 1 0 40%; 
  flex-basis: 40%; start field at 40% wide
  flex-shrink: 0; don't shrink smaller than starting value
  flex-grow: 1; give it 1 (equal) share of any available extra width on it's line

align-items vs. align-content

align-content sets how multiple lines are spaced apart.

align-content determines the spacing between lines, while align-items determines how the items as a whole are aligned within the container. When there is only one line, align-content has no effect

  • align-items interferes with flex items in a row while align-content interferes with rows themselves. align-content comes into play when you have flex-wrap: wrap defined and your items are wrapped (i.e. are on multiple rows)
  • align-content is for multi-line flex containers, it has no effect when flex items are on the same line.
  • align-items (cross-axis) is the opposite of justify-content (main-axis), it aligns flex items inside a flex container


make all flex-items the same width

Give it a flex value. flex can take three values: flex-grow, flex-shrink, flex-basis

.el {
  flex: 1; /* Three values: flex-grow (number) | flex-shrink (number) | flex-basis (width/height)*/
  • Flexbox form (that is two fields wide on bigger screen but 1 field wide on smaller ones, should take about 5 lines of CSS code)
  • First link flush left, last link flush right, equal space in between (a regular nav bar). Easy peasy with flex