Theflexible box layout module (usually referred to as flexbox) is a one-dimensional layout model for distributing space between items and includes numerous alignment capabilities. This article gives an outline of the main features of flexbox, which we will explore in more detail in the rest of these guides.

When we describe flexbox as being one-dimensional we are describing the fact that flexbox deals with layout in one dimension at a time — either as a row or as a column. This can be contrasted with the two-dimensional model ofCSS Grid Layout, which controls columns and rows together.

The two axes of flexbox

When working with flexbox you need to think in terms of two axes — themain axis and thecross axis. Themain axis is defined by theflex-direction property, and thecross axis runs perpendicular to it. Everything we do with flexbox refers back to these axes, so it is worth understanding how they work from the outset.

The main axis

Themain axis is defined byflex-direction, which has four possible values:

• row
• row-reverse
• column
• column-reverse

Should you chooserow orrow-reverse, your main axis will run along the row in theinline direction.

Choosecolumn orcolumn-reverse and your main axis will run in theblock direction, from the top of the page to the bottom.

The cross axis

Thecross axis runs perpendicular to the main axis. Therefore, if yourflex-direction (main axis) is set torow orrow-reverse the cross axis runs down the columns.

If your main axis iscolumn orcolumn-reverse then the cross axis runs along the rows.

Start and end lines

Another vital area of understanding is how flexbox makes no assumption about the writing mode of the document. Flexbox doesn't just assume that all lines of text start at the top left of a document and run towards the right-hand side, with new lines appearing one under the other. Rather, it supports all writing modes, like otherlogical properties and values.

You canread more about the relationship between flexbox and writing modes in a later article; however, the following description should help explain why we do not talk about left and right and top and bottom when we describe the direction that our flex items flow in.

If theflex-direction isrow and I am working in English, then the start edge of the main axis will be on the left, the end edge on the right.

If I were to work in Arabic, then the start edge of my main axis would be on the right and the end edge on the left.

In both cases the start edge of the cross-axis is at the top of the flex container and the end edge at the bottom, as both languages have a horizontal writing mode.

After a while, thinking about start and end rather than left and right becomes natural, and will be useful to you when dealing with other layout methods such as CSS Grid Layout which follow the same patterns.

The flex container

An area of a document that is laid out using flexbox is called aflex container. To create aflex container, set the area'sdisplay property toflex. When we do this, the direct children of that container becomeflex items. You can explicitly control whether the container itself is displayed inline or in block formatting context usinginline flex orinline-flex for inline flex containers orblock flex orflex for block level flex containers.

Initial values

As with all properties in CSS, some initial values are defined, so the contents of a new flex container will behave in the following way:

• Items display in a row (theflex-direction property's default value isrow).
• The items start from the start edge of the main axis.
• The items do not stretch on the main dimension but can shrink (a flex-item'sflex-grow property's default value is0 and itsflex-shrink property's default value is1).
• The items will stretch to fill the size of the cross-axis (thealign-items property's default value isstretch).
• The flex-item'sflex-basis property's default value isauto. This means that, in each case, it will be equal to the flex itemwidth in horizontal writing mode, and the flex itemheight in vertical writing mode. If the correspondingwidth/height is also set toauto, theflex-basiscontent value is used instead.
• All the items will be in a single row (theflex-wrap property's default value isnowrap), overflowing their container if their combinedwidth/height exceeds the containing elementwidth/height.

The result of this is that your items will all line up in a row, using the size of the content as their size in the main axis. If there are more items than can fit in the container, they will not wrap but will instead overflow. If some items are taller than others, all items will stretch along the full length of the cross-axis.

You can see in the live sample below how this looks. Click "Play" to open the example in the MDN Playground and edit the items or add new items to try out the initial behavior of flexbox:

<div>  <div>One</div>  <div>Two</div>  <div>Three <br />has <br />extra <br />text</div></div>

.box > * {  border: 2px solid rgb(96 139 168);  border-radius: 5px;  background-color: rgb(96 139 168 / 0.2);}.box {  border: 2px dotted rgb(96 139 168);  display: flex;}

Changing flex-direction

Adding theflex-direction property to the flex container allows us to change the direction in which our flex items display. Settingflex-direction: row-reverse will keep the items displaying along the row, however the start and end lines are switched.

If we changeflex-direction tocolumn the main axis switches and our items now display in a column. Setcolumn-reverse and the start and end lines are again switched.

The live sample below hasflex-direction set torow-reverse. Try the other values —row,column andcolumn-reverse — to see what happens to the content.

<div>  <div>One</div>  <div>Two</div>  <div>Three</div></div>

.box > * {  border: 2px solid rgb(96 139 168);  border-radius: 5px;  background-color: rgb(96 139 168 / 0.2);}.box {  border: 2px dotted rgb(96 139 168);  display: flex;  flex-direction: row-reverse;}

Multi-line flex containers with flex-wrap

While flexbox is a one dimensional model, it is possible to make flex items wrap across multiple lines. If you do this, you should consider each line as a new flex container. Any space distribution will happen across each line, without reference to the previous or subsequent lines.

To cause wrapping behavior add the propertyflex-wrap with a value ofwrap. Now, if your items are too large to all display in one line, they will wrap onto another line. The live sample below contains items that have been given awidth. The total width of the items is too wide for the flex container. Asflex-wrap is set towrap, the items wrap across multiple lines. If you set it tonowrap, which is the initial value, they will shrink to fit the container. They shrink because they are using initial flexbox values, includingflex-shrink: 1, that allows items to shrink. Usingnowrap would cause anoverflow if the items were not able to shrink, or could not shrink small enough to fit.

<div>  <div>One</div>  <div>Two</div>  <div>Three</div></div>

.box > * {  border: 2px solid rgb(96 139 168);  border-radius: 5px;  background-color: rgb(96 139 168 / 0.2);  width: 200px;}.box {  width: 500px;  border: 2px dotted rgb(96 139 168);  display: flex;  flex-wrap: wrap;}

Find out more about wrapping flex items in the guideMastering wrapping of flex items.

The flex-flow shorthand

You can combine the two propertiesflex-direction andflex-wrap into theflex-flow shorthand.

In the live sample below, try changing the first value to one of the allowable values forflex-direction -row,row-reverse,column orcolumn-reverse, and also change the second towrap andnowrap.

<div>  <div>One</div>  <div>Two</div>  <div>Three</div></div>

.box > * {  border: 2px solid rgb(96 139 168);  border-radius: 5px;  background-color: rgb(96 139 168 / 0.2);  width: 200px;}.box {  width: 500px;  border: 2px dotted rgb(96 139 168);  display: flex;  flex-flow: row wrap;}

Properties applied to flex items

To control the inline-size of each flex item, we target them directly via three properties:

• flex-grow
• flex-shrink
• flex-basis

We will take a brief look at these properties below, but if you want more comprehensive information, take a look at theControlling ratios of flex items on the main axis guide.

Before we can make sense of these properties we need to consider the concept ofavailable space. What we are doing when we change the value of these flex properties is to change the way that available space is distributed amongst our items. This concept of available space is also important when we come to look at aligning items.

If we have three 100 pixel-wide items in a container which is 500 pixels wide, then the space we need to lay out our items is 300 pixels. This leaves 200 pixels of available space. If we don't change the initial values then flexbox will put that space after the last item.

If we instead would like the items to grow and fill the space, then we need to have a method of distributing the leftover space between the items. Theflex properties that we apply to the items themselves, enable dictating how that available space should be distributed among the sibling flex items.

The flex-basis property

Theflex-basis is what defines the size of that item in terms of the space it leaves as available space. The initial value of this property isauto — in this case the browser looks to see if the item has a size. In the example above, all of the items have a width of 100 pixels. This is used as theflex-basis.

If the items don't have a size then the content's size is used as the flex-basis. This is why when we just declaredisplay: flex on the parent to create flex items, the items all move into a row and take only as much space as they need to display their contents.

The flex-grow property

With theflex-grow property set to a positive integer, if there is available space, the flex item can grow along the main axis from itsflex-basis. Whether the item stretches to take up all the available space on that axis, or just a portion of the available space depends on if the other items are allowed to grow too and the value of theirflex-grow properties.

Each item with a positive value consumes a portion of any available space based on theirflex-grow value. If we gave all of our items in the example above aflex-grow value of 1 then the available space in the flex container would be equally shared between our items and they would stretch to fill the container on the main axis. If we give our first item aflex-grow value of 2, and the other items a value of 1 each, there are a total of 4 parts; 2 parts of the available space will be given to the first item (100px out of 200px in the case of the example above) and 1 part each the other two (50px each out of the 200px total).

The flex-shrink property

Where theflex-grow property deals with adding space in the main axis, theflex-shrink property controls how it is taken away. If we do not have enough space in the container to lay out our items, andflex-shrink is set to a positive integer, then the item can become smaller than theflex-basis. As withflex-grow, different values can be assigned in order to cause one item to shrink faster than others — an item with a higher value set forflex-shrink will shrink faster than its siblings that have lower values.

An item can shrink down to itsmin-content size. This minimum size is taken into account while working out the actual amount of shrinkage that will happen, which means thatflex-shrink has the potential to appear less consistent thanflex-grow in behavior. We'll therefore take a more detailed look at how this algorithm works in the articleControlling ratios of items along the main axis.

Shorthand values for the flex properties

You will very rarely see theflex-grow,flex-shrink, andflex-basis properties used individually; instead they are combined into theflex shorthand. Theflex shorthand allows you to set the three values in this order —flex-grow,flex-shrink,flex-basis.

The live sample below allows you to test out the different values of the flex shorthand; remember that the first value isflex-grow. Giving this a positive value means the item can grow. The second isflex-shrink — with a positive value the items can shrink, but only if their total values overflow the main axis. The final value isflex-basis; this is the value the items are using as their base value to grow and shrink from.

<div>  <div>One</div>  <div>Two</div>  <div>Three</div></div>

.box > * {  border: 2px solid rgb(96 139 168);  border-radius: 5px;  background-color: rgb(96 139 168 / 0.2);}.box {  border: 2px dotted rgb(96 139 168);  display: flex;}.one {  flex: 1 1 auto;}.two {  flex: 1 1 auto;}.three {  flex: 1 1 auto;}

There are also some predefined shorthand values which cover most of the use cases. You will often see these used in tutorials, and in many cases these are all you will need to use. The predefined values are as follows:

• flex: initial
• flex: auto
• flex: none
• flex: <positive-number>

Theinitial value is aCSS-wide keyword that represents the initial value for a property. Settingflex: initial resets the item to theinitial values of the three longhand properties, which is the same asflex: 0 1 auto. The initial value offlex-grow is0, so items will not grow larger than theirflex-basis size. The initial value offlex-shrink is1, so items can shrink if they need to rather than overflowing. The initial value offlex-basis isauto. Items will either use any size set on the item in the main dimension, or they will get their size from the content size.

Usingflex: auto is the same as usingflex: 1 1 auto; this is similar toflex: initial, except that the items can grow and fill the container as well as shrink if needed.

Usingflex: none will create fully inflexible flex items. It is as if you wroteflex: 0 0 auto. The items cannot grow or shrink and will be laid out using flexbox with aflex-basis ofauto.

The shorthand you often see in tutorials isflex: 1 orflex: 2 and so on. This is the same as writingflex: 1 1 0 orflex: 2 1 0 and so on, respectively. The items get minimum size due toflex-basis: 0 and then proportionally grow to fill the available space. In this case, theflex-shrink value of1 is redundant because the items start with minimum size — they're not given any size that could cause them to overflow the flex container.

Try these shorthand values in the live sample below.

<div>  <div>One</div>  <div>Two</div>  <div>Three</div></div>

.box > * {  border: 2px solid rgb(96 139 168);  border-radius: 5px;  background-color: rgb(96 139 168 / 0.2);}.box {  border: 2px dotted rgb(96 139 168);  display: flex;}.one {  flex: 1;}.two {  flex: 1;}.three {  flex: 1;}

Alignment, justification and distribution of free space between items

A key feature of flexbox is the ability to align and justify items on the main- and cross-axes, and to distribute space between flex items. Note that these properties are set on the flex container, not on the items themselves.

align-items

Thealign-items property aligns all the flex items on the cross axis.

The initial value for this property isstretch and is why flex items stretch to the height of the flex container by default (or the width ifflex-direction is set tocolumn orcolumn-reverse). This height may come from the tallest item in the container or the size set on the flex container itself.

You could instead setalign-items toflex-start, or simplystart, in order to make the items line up at the start of the flex container,flex-end, or justend, to align them to the end, orcenter to align them in the center. Try this in the live sample — I have given the flex container a height in order that you can see how the items can be moved around inside the container. See what happens if you set the value of align-items to:

• stretch
• flex-start
• flex-end
• start
• end
• center
• baseline
• last baseline

<div>  <div>One</div>  <div>Two</div>  <div>Three <br />has <br />extra <br />text</div></div>

.box > * {  border: 2px solid rgb(96 139 168);  border-radius: 5px;  background-color: rgb(96 139 168 / 0.2);}.box {  width: 500px;  height: 130px;  border: 2px dotted rgb(96 139 168);  display: flex;  align-items: flex-start;}

Thealign-items is set on the flex container and impacts all the flex items. If you want to align a flex item differently from others, you can set thealign-self on the flex item.

justify-content

Thejustify-content property is used to align the items on the main axis, the direction in whichflex-direction has set the flow. The initial value isflex-start which will line the items up at the start edge of the container, but you could also set the value toflex-end to line them up at the end, orcenter to line them up in the center.

You can also use the valuespace-between to take all the spare space after the items have been laid out, and share it out evenly between the items so there will be an equal amount of space between each item. To cause an equal amount of space on the right and left (or top and bottom for columns) of each item use the valuespace-around. Withspace-around, items have a half-size space on either end. Or, to cause items to have equal space around them use the valuespace-evenly. Withspace-evenly, items have a full-size space on either end.

Try the following values ofjustify-content in the live sample:

• start
• end
• left
• right
• normal
• flex-start
• flex-end
• center
• space-around
• space-between
• space-evenly
• stretch

<div>  <div>One</div>  <div>Two</div>  <div>Three</div></div>

.box > * {  border: 2px solid rgb(96 139 168);  border-radius: 5px;  background-color: rgb(96 139 168 / 0.2);}.box {  border: 2px dotted rgb(96 139 168);  display: flex;  justify-content: flex-start;}

The articleAligning items in a flex container explores these properties in more depth, in order to have a better understanding of how they work. These basic examples, however, are useful in the majority of use cases.

justify-items

Thejustify-items property is ignored in flexbox layouts.

place-items and place-content

Theplace-items property is a shorthand property foralign-items andjustify-items. If set on a flex container, it will set the alignment but not the justification, asjustify-items is ignored in flexbox.

There is another shorthand property,place-content, that defines thealign-content andjustify-content properties. Thealign-content property only effects flex containers that wrap, and is discussed inAligning items in a flex container.

Next steps

After reading this article you should have an understanding of the basic features of flexbox. In the next article, we will look athow this specification relates to other parts of CSS.