A Complete Guide to Flexbox | CSS-Tricks - CSS-Tricks
Excerpt
Our comprehensive guide to CSS flexbox layout. This complete guide explains everything about flexbox, focusing on all the different possible properties for the parent element (the flex container) and the child elements (the flex items). It also includes history, demos, patterns, and a browser support chart.
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The Flexbox Layout (Flexible Box) module ( as of October 2017) aims at providing a more efficient way to lay out, align and distribute space among items in a container, even when their size is unknown and/or dynamic (thus the word “flex”).
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.
Most importantly, the flexbox layout is direction-agnostic as opposed to the regular layouts (block which is vertically-based and inline which is horizontally-based). While those work well for pages, they lack flexibility (no pun intended) to support large or complex applications (especially when it comes to orientation changing, resizing, stretching, shrinking, etc.).
Note: Flexbox layout is most appropriate to the components of an application, and small-scale layouts, while the layout is intended for larger scale layouts.
Basics and terminology
Since flexbox is a whole module and not a single property, it involves a lot of things including its whole set of properties. Some of them are meant to be set on the container (parent element, known as “flex container”) whereas the others are meant to be set on the children (said “flex items”).
If “regular” layout is based on both block and inline flow directions, the flex layout is based on “flex-flow directions”. Please have a look at this figure from the specification, explaining the main idea behind the flex layout.
Items will be laid out following either the main axis (from main-start to main-end) or the cross axis (from cross-start to cross-end).
main axis – The main axis of a flex container is the primary axis along which flex items are laid out. Beware, it is not necessarily horizontal; it depends on the flex-direction property (see below).
main-start | main-end – The flex items are placed within the container starting from main-start and going to main-end.
Flexbox properties
Properties for the Parent
(flex container)
display
This defines a flex container; inline or block depending on the given value. It enables a flex context for all its direct children.
Note that CSS columns have no effect on a flex container.
flex-direction
This establishes the main-axis, thus defining the direction flex items are placed in the flex container. Flexbox is (aside from optional wrapping) a single-direction layout concept. Think of flex items as primarily laying out either in horizontal rows or vertical columns.
row (default): left to right in ltr; right to left in rtl
row-reverse: right to left in ltr; left to right in rtl
flex-wrap
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 this property.
nowrap (default): all flex items will be on one line
wrap: flex items will wrap onto multiple lines, from top to bottom.
There are some .
flex-flow
This is a shorthand for the flex-direction and flex-wrap properties, which together define the flex container’s main and cross axes. The default value is row nowrap.
justify-content
This defines the alignment along the main axis. It helps distribute extra free space leftover when either all the flex items on a line are inflexible, or are flexible but have reached their maximum size. It also exerts some control over the alignment of items when they overflow the line.
flex-start (default): items are packed toward the start of the flex-direction.
flex-end: items are packed toward the end of the flex-direction.
Note that that browser support for these values is nuanced. For example, space-between never got support from some versions of Edge, and start/end/left/right aren’t in Chrome yet. MDN . The safest values are flex-start, flex-end, and center.
There are also two additional keywords you can pair with these values: safe and unsafe. Using safe ensures that however you do this type of positioning, you can’t push an element such that it renders off-screen (e.g. off the top) in such a way the content can’t be scrolled too (called “data loss”).
align-items
This defines the default behavior for how flex items are laid out along the cross axis on the current line. Think of it as the justify-content version for the cross-axis (perpendicular to the main-axis).
stretch (default): stretch to fill the container (still respect min-width/max-width)
flex-start / start / self-start: items are placed at the start of the cross axis. The difference between these is subtle, and is about respecting the flex-direction rules or the writing-mode
The safe and unsafe modifier keywords can be used in conjunction with all the rest of these keywords (although note ), and deal with helping you prevent aligning elements such that the content becomes inaccessible.
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 only takes effect on multi-line flexible containers, where flex-wrap is set to either wrap or wrap-reverse). A single-line flexible container (i.e. where flex-wrap is set to its default value, no-wrap) will not reflect align-content.
normal (default): items are packed in their default position as if no value was set.
flex-start / start: items packed to the start of the container. The (more supported) flex-start honors the flex-direction while start honors the
The safe and unsafe modifier keywords can be used in conjunction with all the rest of these keywords (although note ), and deal with helping you prevent aligning elements such that the content becomes inaccessible.
gap, row-gap, column-gap
explicitly controls the space between flex items. It applies that spacing only between items not on the outer edges.
The behavior could be thought of as a minimum gutter, as if the gutter is bigger somehow (because of something like justify-content: space-between;) then the gap will only take effect if that space would end up smaller.
It is not exclusively for flexbox, gap works in grid and multi-column layout as well.
Properties for the Children
(flex items)
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.
Items with the same order revert to source order.
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.
If all items have flex-grow set to 1, the remaining space in the container will be distributed equally to all children. If one of the children has a value of 2, that child would take up twice as much of the space either one of the others (or it will try, at least).
Negative numbers are invalid.
flex-shrink
This defines the ability for a flex item to shrink if necessary.
Negative numbers are invalid.
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. The auto keyword means “look at my width or height property” (which was temporarily done by the main-size keyword until deprecated). The content keyword means “size it based on the item’s content” – this keyword isn’t well supported yet, so it’s hard to test and harder to know what its brethren max-content, min-content, and fit-content do.
If set to 0, the extra space around content isn’t factored in. If set to auto, the extra space is distributed based on its flex-grow value.
flex
This is the shorthand for flex-grow, flex-shrink and flex-basis combined. The second and third parameters (flex-shrink and flex-basis) are optional. The default is 0 1 auto, but if you set it with a single number value, like flex: 5;, that changes the flex-basis to 0%, so it’s like setting flex-grow: 5; flex-shrink: 1; flex-basis: 0%;.
It is recommended that you use this shorthand property rather than set the individual properties. The shorthand sets the other values intelligently.
align-self
This allows the default alignment (or the one specified by align-items) to be overridden for individual flex items.
Please see the align-items explanation to understand the available values.
Note that float, clear and vertical-align have no effect on a flex item.
Prefixing Flexbox
Flexbox requires some vendor prefixing to support the most browsers possible. It doesn’t just include prepending properties with the vendor prefix, but there are actually entirely different property and value names. This is because the Flexbox spec has changed over time, creating an versions.
Perhaps the best way to handle this is to write in the new (and final) syntax and run your CSS through , which handles the fallbacks very well.
Alternatively, here’s a Sass @mixin to help with some of the prefixing, which also gives you an idea of what kind of things need to be done:
Let’s start with a very very simple example, solving an almost daily problem: perfect centering. It couldn’t be any simpler if you use flexbox.
This relies on the fact a margin set to auto in a flex container absorb extra space. So setting a margin of auto will make the item perfectly centered in both axes.
Now let’s use some more properties. Consider a list of 6 items, all with fixed dimensions, but can be auto-sized. We want them to be evenly distributed on the horizontal axis so that when we resize the browser, everything scales nicely, and without media queries.
Done. Everything else is just some styling concern. Below is a pen featuring this example. Be sure to go to CodePen and try resizing your windows to see what happens.
Let’s try something else. Imagine we have a right-aligned navigation element on the very top of our website, but we want it to be centered on medium-sized screens and single-columned on small devices. Easy enough.
Let’s try something even better by playing with flex items flexibility! What about a mobile-first 3-columns layout with full-width header and footer. And independent from source order.
Browser support
Desktop
Chrome
Firefox
IE
Edge
Safari
Mobile / Tablet
Android Chrome
Android Firefox
Android
iOS Safari
Flexbox is certainly not without its bugs. The best collection of them I’ve seen is Philip Walton and Greg Whitworth’s . It’s an open-source place to track all of them, so I think it’s best to just link to that.
Related properties
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