CSS Flexbox arranges items along one axis at a time: a row or a column. Set display: flex on a parent to make its in-flow children flex items, then use axis-aware properties to control their size, spacing, alignment, and wrapping. This guide builds a navigation bar and a wrapping card group, explains the flex shorthand, and shows when CSS Grid is a better fit.
How Flexbox works: container, items, and axes
A flex layout has a parent called the flex container and its in-flow children called flex items. The container establishes the layout context; properties on it arrange the items, while properties on individual items can adjust their sizing or alignment.
The flex-direction property establishes the main axis. The cross axis runs perpendicular to it. The default direction is row, so the main axis follows the row direction and the cross axis runs across it. With column, those roles rotate. Thinking in terms of axes is more reliable than saying “horizontal” and “vertical,” especially when direction changes or writing modes matter.
.toolbar {
display: flex;
flex-direction: row;
}
.toolbar--stacked {
flex-direction: column;
}In the first example, the toolbar’s children are flex items arranged along a row. In the second, they are arranged along a column. The properties that align items follow the axes set by flex-direction.
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For background and the full learning path, see MDN’s Flexbox guide and its flexible box layout reference. W3C describes Flexbox as “a CSS box model optimized for user interface design” in the CSS Flexible Box Layout Module Level 1. The cited W3C document is a Candidate Recommendation Draft dated 14 October 2025; W3C notes that the draft is work in progress and may be updated, replaced, or obsoleted.
Build a one-line navigation bar
Start with semantic HTML and apply Flexbox to the element whose children should be laid out together. Here, the navigation element is the container and its links are the flex items.
<nav class="site-nav" aria-label="Main navigation">
<a href="/">Home</a>
<a href="/guides/">Guides</a>
<a href="/about/">About</a>
</nav>.site-nav {
display: flex;
flex-direction: row;
align-items: center;
gap: 1rem;
padding: 1rem;
border-bottom: 1px solid #ccc;
}
.site-nav a {
color: #164b8b;
}display: flex makes the navigation a flex container. gap creates consistent space between its items without adding margins to each link. Here, align-items: center aligns items on the cross axis. Because the direction is row, that is the perpendicular axis, not the axis along which the links are placed.
Align and space items along the axes
Use justify-content to distribute items along the main axis. Use align-items to align all items on the cross axis, or align-self to override that cross-axis alignment for one item. The axes depend on flex-direction; these properties do not always correspond to horizontal and vertical alignment.
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| Property | Applies to | Useful values | What it controls |
|---|---|---|---|
justify-content |
Flex container | flex-start, center, space-between, space-around, space-evenly |
How items are positioned or distributed along the main axis. |
align-items |
Flex container | stretch, flex-start, center, flex-end |
Default cross-axis alignment for the container’s items. |
align-self |
Flex item | auto, stretch, flex-start, center, flex-end |
Cross-axis alignment for an individual item. |
gap |
Flex container | A length such as 1rem, or row and column gaps |
Space between items or flex lines without adding space at the container’s outside edges. |
For example, a toolbar can keep one item at the start and push another to the far end by letting the middle space expand:
.toolbar {
display: flex;
align-items: center;
gap: 1rem;
}
.toolbar__spacer {
flex-grow: 1;
}The spacer grows to take available positive free space, separating the other items. If you instead need equal distribution across the main axis, try justify-content: space-between. Choose the behavior that matches the design rather than adding arbitrary margins to individual items.
Wrap a group of cards onto additional lines
Flex items remain on one line by default. Set flex-wrap: wrap when a group should continue on additional lines as space runs out. Each resulting flex line is handled as a line in the cross direction; wrapping does not turn Flexbox into a layout system that coordinates both rows and columns as a grid.
.card-list {
display: flex;
flex-wrap: wrap;
gap: 1rem;
}
.card {
flex: 1 1 14rem;
}This gives each card a starting basis of 14rem, allows it to grow when there is room, and allows it to shrink when needed. When the line cannot accommodate the items at their preferred sizes, items can move onto another flex line. The exact result also depends on the available container width and other size constraints.
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flex-flow is shorthand for setting both direction and wrapping. For instance, flex-flow: row wrap is equivalent to flex-direction: row; flex-wrap: wrap;.
Understand flexible sizing and the flex shorthand
The three sizing components are flex-grow, flex-shrink, and flex-basis. The basis is the item’s starting main-axis size before free space is distributed. If there is positive free space, grow factors determine relative shares; if there is not enough space, shrink factors control reduction. The final size can also be constrained by other sizing rules on the item.
flex-growcontrols how an item shares positive free space with other growable items. A value of0means it does not grow on that basis; a positive value makes it eligible to grow.flex-shrinkcontrols how an item participates in reducing sizes when the line has insufficient space. A value of0opts it out of shrinking; a positive value permits shrinking.flex-basissets the starting size along the main axis. It can be a length such as12rem, a percentage, orauto.
The flex shorthand sets these components together. Common forms are:
| Declaration | Equivalent factors | Typical effect |
|---|---|---|
flex: initial |
0 1 auto |
Items use their automatic basis, can shrink, and do not grow into free space. |
flex: auto |
1 1 auto |
Items use their automatic basis and can both grow and shrink. |
flex: none |
0 0 auto |
Items use their automatic basis and neither grow nor shrink. |
flex: 2 |
Numeric factor with a zero basis | Sets a grow factor of 2; the shorthand’s omitted components use the specification’s numeric-factor behavior. |
flex: 1 1 12rem |
Grow 1, shrink 1, basis 12rem | Starts at the stated basis and can grow or shrink with the line. |
The first three mappings and the role of grow and shrink follow the W3C specification’s basic values and sizing definitions in the Flexbox specification. Prefer an explicit three-part shorthand when the intended starting size matters; it makes the sizing behavior easier to read and debug.
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Center a component with Flexbox
To center a component in both directions, make its parent the flex container and center on each axis. Give the container a height or minimum height if it needs a visible area in which to center the child.
.stage {
display: flex;
justify-content: center;
align-items: center;
min-height: 20rem;
}
.dialog {
width: min(100%, 28rem);
}With the default row direction, justify-content centers along the main axis and align-items centers along the cross axis. If you switch the parent to flex-direction: column, the main and cross axes swap roles, so the same property names center along different physical directions.
Choose Flexbox or Grid
Flexbox is one-dimensional: it arranges and sizes items along a main axis, with a cross axis for alignment and wrapped lines. CSS Grid is two-dimensional: it can control rows and columns together. Choose based on the relationships the design needs, not on which tool is newer or more familiar. MDN explains this distinction in its CSS layout guide; its layout cookbook compares practical layout patterns.
| Question | Flexbox is a natural fit when… | Grid is a natural fit when… |
|---|---|---|
| What is the relationship? | Items primarily form one row or one column, such as navigation links or toolbar controls. | The design coordinates rows and columns as a two-dimensional layout. |
| What should sizing respond to? | Items should share or give up space along one direction. | Rows and columns need controlled tracks or coordinated placement. |
| What should happen when space runs out? | Items should wrap into additional flex lines. | Items need explicit two-dimensional placement or aligned row and column structure. |
Both tools can be appropriate for some patterns. If the layout’s key requirement is aligned columns across multiple rows, Grid is often clearer than trying to coordinate independently wrapped flex lines. If the content is a single flow of items that can wrap naturally, Flexbox is often simpler.
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Debug a Flexbox layout in browser DevTools
- Select the intended parent. In the Elements or Inspector panel, verify that the element you expect to control the layout has
display: flexordisplay: inline-flex. - Confirm the items. Check which in-flow children are direct children of that container. A nested element is not a flex item of an outer container unless it is itself a direct child.
- Read the axes. Inspect the computed
flex-direction. Determine the main and cross axes from that value before changing alignment properties. - Check available space and wrapping. Inspect the container’s size, item bases, and
flex-wrap. A line that has no free space cannot distribute positive space through growth; insufficient room can cause shrinking or wrapping. - Change one value at a time. Toggle
justify-content,align-items,gap, or the relevantflexcomponent in the Styles panel, then observe the result before editing your stylesheet. - Check computed sizing and constraints. Look for explicit widths, min/max sizes, and content that resists fitting; these can affect the outcome even when the flex factors seem reasonable.
MDN’s CSS debugging guide covers browser DevTools and compatibility information. Use the browser’s own computed-style view to diagnose the page and check compatibility for the browsers your project supports.
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Frequently Asked Questions
Does setting display: flex affect every descendant?
No. The flex items are the container’s in-flow children, not every nested descendant.
Can Flexbox make a true two-dimensional grid?
No. Flexbox lays out one axis at a time; use CSS Grid when the design needs coordinated row and column control.
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