---
title: "CSS: Flexbox"
slug: "css-flexbox"
category: "frontend"
tags: ["frontend", "css", "flexbox", "layout", "gap"]
status: "stable"
last_updated: 2026-10-01
summary: "Flex for one-dimensional layout: main versus cross axis, gap over margins, the grow/shrink/basis shorthand, and min-width: 0 for children that must shrink."
related: ["[[frontend/css]]", "[[frontend/css-grid]]", "[[frontend/css-custom-properties]]", "[[frontend/tailwind]]", "[[frontend/accessibility]]", "[[frontend/css-container-queries]]"]
---

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## Overview

Flexbox lays out a single row or column whose children grow, shrink, and align along one axis: nav bars, toolbars, button rows, input groups. When rows and columns must align with each other, use [[frontend/css-grid]]. The umbrella page is [[frontend/css]].

## Use flex for one dimension and grid for two

A row of buttons or a media object is flex; a page shell with sidebar, header, and footer is grid. The test is whether items on different rows must line up by column. If you use `flex-wrap` plus manual widths to fake a grid, write `grid-template-columns: repeat(auto-fill, minmax(...))` instead.

## Name the axes

`flex-direction` sets the main axis; the cross axis is perpendicular.

- `justify-content` aligns on the main axis.
- `align-items` aligns each item on the cross axis.
- `align-content` aligns wrapped lines on the cross axis and does nothing without `flex-wrap: wrap`.

If `align-items: center` seems to stop working after you change direction, the axes swapped.

## Use `gap` instead of margins

`gap` on flex containers has been Baseline widely available since October 2023. It never leaks past the last child, needs no `:last-child` overrides, and survives wrapping.

```css
.toolbar { display: flex; gap: 0.5rem; align-items: center; }
```

## Read `flex` as grow, shrink, basis

```css
.sidebar { flex: 0 0 240px; }  /* fixed width */
.main    { flex: 1 1 0; min-width: 0; }  /* fill the rest */
.button  { flex: none; }       /* size to content */
```

- `flex: 1` is `1 1 0%`: fill remaining space and ignore intrinsic size.
- `flex: auto` is `1 1 auto`: fill, but start from intrinsic size.
- `flex: none` is `0 0 auto`: locked to content.

## Set `min-width: 0` on children that must shrink

A flex item defaults to `min-width: auto`, so it will not shrink below its content. A long word, a wide table, or a `pre` inside a `flex: 1` child then overflows the container. Set `min-width: 0` (and `min-height: 0` for column overflow), then give the content its own `overflow`.

```css
.main pre { overflow-x: auto; }
```

## Avoid common pitfalls

- **Centering with `place-items`.** On a flex container only the `align-items` half applies. Use `justify-content` plus `align-items`, or switch to grid.
- **Percentage heights in a column.** They need a parent with a definite height; define `height` or `min-height` on the container.
- **Wrapping on a fixed container.** Items wrap by basis plus content, not by container width; set `flex-basis` explicitly.
- **`order` and tab order.** `order` and `row-reverse` change only the visual order; keyboard focus follows the DOM. See [[frontend/accessibility]].

## Related

- [[frontend/css]]
- [[frontend/css-grid]]
- [[frontend/css-custom-properties]]
- [[frontend/tailwind]]
- [[frontend/accessibility]]
- [[frontend/css-container-queries]]