How Flexbox Align Items & Justify Content Sandbox Works

Flexbox is built around two axes: the main axis (defined by flex-direction) and the cross axis (perpendicular to the main axis). The sandbox lets you switch direction, then experiment with alignment controls to see how items distribute space and line up within the container. The generated snippet includes the container rule, a simple item rule, and an optional highlighted item with align-self, so you can reproduce what you see with minimal copy/paste.

The tool’s preview intentionally uses a “boxy” design with visible borders and numbered items. That makes subtle alignment changes easy to spot, especially when you experiment with stretching, wrapping, and different gaps. You can also resize the container to simulate real constraints: toolbars, card footers, navigation bars, or stacked mobile layouts.

Step-by-step

• 1) Choose a direction: Pick row or column (and the reverse variants) to set the main axis.
• 2) Set main-axis distribution: Adjust justify-content to control how free space is placed along the main axis.
• 3) Set cross-axis alignment: Adjust align-items to align items as a group on the cross axis.
• 4) Try wrapping: Switch flex-wrap to wrap and explore align-content for multi-line layouts.
• 5) Tune spacing and sizing: Change gap, container size, padding, and item size to match real UI constraints.
• 6) Override one item: Pick a highlighted item index and set align-self to see how per-item alignment differs.
• 7) Generate code: Click Generate to produce a copy-ready snippet you can paste into your stylesheet.

Quick mental model

• Main axis: Think “flow direction.” In row, it is left-to-right; in column, it is top-to-bottom.
• justify-content: Controls distribution of extra space along the main axis (packing, spacing, or centering).
• Cross axis: Think “perpendicular alignment.” In row, it is vertical; in column, it is horizontal.
• align-items: Controls alignment of items as a group on the cross axis (start/end/center/stretch/baseline).
• align-self: Overrides align-items for a single item, useful for badges, icons, or a single CTA.
• align-content: Only relevant when there are multiple lines (wrap). It aligns the set of lines on the cross axis.

Key Features

Live preview with practical defaults

The preview starts with realistic dimensions and a set of numbered items so you can see alignment differences immediately. Defaults are chosen to demonstrate the most common UI scenarios: centered content, consistent spacing, and predictable item sizing. This means you don’t need to type anything before learning starts—you can just toggle values and watch the layout respond.

Focused on align-items and justify-content

Many developers learn flexbox by copying snippets without understanding the axis model. This tool keeps the spotlight on justify-content (main axis) and align-items (cross axis) so you can build intuition quickly. Each value is paired with a short explanation that connects the code to what you see: “pack to the start,” “distribute evenly,” “stretch to fill,” and so on.

You can also learn the “why” behind common mistakes. For example, developers often try to vertically center items with justify-content in a row container, forgetting that vertical centering is the cross axis and needs align-items. Switching to flex-direction: column flips the axes and flips which property feels like the “centering” control—this sandbox makes that flip obvious.

Wrap and multi-row behavior explained

When wrapping is enabled, flex items can form multiple lines. At that point, align-content becomes relevant because it distributes space between lines. The sandbox makes this relationship obvious by letting you toggle wrapping and instantly see the difference between aligning items within a line versus aligning lines within the container.

This is especially helpful for “chip” components, tag clouds, filters, and responsive button groups. Those layouts can look perfect at one width and awkward at another if line spacing and line alignment are not configured intentionally.

Built-in value glossary for fast recall

Flexbox values are short but not always intuitive at a glance. The tool includes a quick reference for the most-used values: flex-start, flex-end, center, space-between, space-around, and space-evenly for distribution; plus stretch and baseline for alignment. This reduces the need to jump between documentation tabs when you just want to confirm what “space-around” does compared to “space-evenly.”

Exportable CSS + HTML snippet

The generated output is designed to be pasted into real projects. You get a clear container rule, an item rule that works with typical UI components, and a highlighted item rule when you want an intentional exception. The snippet uses readable formatting and includes your selected values so you can audit changes later.

Because the output is plain text, it’s easy to drop into a design system, a component story, or a quick reproduction for a bug report. Use it as a starting point, then replace the demo items with your real components.

Optional “extra CSS” for edge cases

Real layouts often need a couple of extra declarations (like border-radius, a background color, or a different border). Add those in the optional field and the tool will append them to the exported container rule so your snippet stays complete. This is handy when you want the preview and the exported snippet to match the “feel” of your application without turning the tool into a full editor.

Use Cases

• Centering content: Find the quickest combination for centering buttons, loaders, cards, or empty states.
• Navigation bars: Compare space-between versus space-evenly for menu items and utility links.
• Tag and chip rows: Enable wrapping and tune gap to create clean, readable clusters of labels.
• Form toolbars: Align inputs and action buttons consistently across different container heights.
• Card grids (flex-based): Explore wrapping behavior before deciding whether a CSS grid is a better fit.
• One-off overrides: Use align-self for a single item (like a “New” badge) without changing the whole row.
• Teaching and documentation: Generate a minimal reproduction snippet for code reviews, docs, or onboarding materials.

In all of these cases, the same principles apply: decide what your main axis is, then distribute and align accordingly. Once you can predict the effect of justify-content and align-items, you’ll build layouts faster and debug them with less trial-and-error. You’ll also be able to communicate intent clearly—“We want to pack items to the start but keep them centered on the cross axis”— which makes collaboration easier.

If you work with a design system, this tool can help you standardize layout primitives. For example, you can define “Toolbar,” “InlineStack,” and “WrapStack” patterns, each with a consistent direction, gap, and alignment. That reduces one-off layout code and creates predictable UI behavior across pages and components.

Optimization Tips

Think in axes, not “left/right” and “top/bottom”

The most common flexbox confusion happens when flex-direction changes. In a row, the main axis is horizontal; in a column, the main axis is vertical. Train yourself to ask: “Do I want to distribute space along the main axis or align along the cross axis?” Then choose justify-content (main) or align-items (cross) accordingly. If something looks “rotated” after switching direction, it often means you’re using the right property for the old axis.

Use gap for spacing, not margins between items

gap is a clean, consistent way to add spacing that works for both wrapping and non-wrapping layouts. It reduces edge cases like extra margin on the last item, and it keeps spacing logic in one place. For responsive layouts, you can even tie gap to a CSS variable or clamp expression in your real project. Start with a readable value in the sandbox, then translate it into your system tokens.

Wrap + align-content is the “two-level” alignment model

With wrapping enabled, alignment happens at two levels: items inside each line (align-items) and the collection of lines inside the container (align-content). If your lines are bunched up at the top with lots of empty space below, align-content is the control you’re missing. Also remember that space-between and friends can create very large gaps between lines in tall containers, so it’s worth testing at multiple container heights.

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