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Flexbox Generator

Professional CSS Flexbox Architect tool. Visualize layouts, calculate flex-grow dynamics, and generate production-ready CSS/Tailwind code with 50+ patterns.

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Category CSS Tools

Layout Architect

Container Properties
10px
Active Child Item
Select an item in the preview to edit properties.
Interactive Preview
→ Main Axis ↓ Cross Axis
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About

The CSS Flexible Box Layout, commonly known as Flexbox, is a layout mode intended to accommodate different screen sizes and different display devices. For frontend architects, the challenge isn't just centering a div; it is understanding the mathematical distribution of free space along the Main Axis and the Cross Axis. This tool goes beyond simple generation - it is a simulation engine that visualizes how the browser rendering engine calculates sizes based on the flex-grow, flex-shrink, and flex-basis algorithms.

Misunderstanding these properties often leads to layout thrashing and overflow issues. This application allows you to prototype complex layouts, audit Tailwind utility classes, and verify responsive behaviors before writing a single line of production code.

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Formulas

The core of Flexbox mathematics lies in the distribution of Free Space (Sfree). The algorithm proceeds in cycles.

Sfree = Wcontainer − n∑i=1 Bi

If Sfree > 0, the items grow. The added size (ΔSi) for item i is calculated as:

ΔSi = Sfree × fgrow,in∑k=1 fgrow,k

If Sfree < 0, the items shrink. The calculation is weighted by the basis size:

Sshrunk = Bi − |Sfree| × fshrink,i × Bin∑k=1 fshrink,k × Bk

Reference Data

PropertyAxisDefaultArchitectural Function
displayN/AblockActivates the flex context. Direct children become flex items.
flex-directionMainrowDefines the direction in which items are placed. Crucial: This rotates the Main Axis and Cross Axis.
justify-contentMainflex-startDistributes extra free space along the Main Axis after flexible lengths and auto margins are resolved.
align-itemsCrossstretchDefines the default behavior for how items are laid out along the Cross Axis (perpendicular to Main).
align-contentCrossnormalAligns a flex container's lines within the flex container when there is extra space in the Cross Axis.
flex-basisItemautoSets the initial main size of the item before free space is distributed. Overrides width/height.
flex-growItem0The factor of remaining free space this item should consume.
flex-shrinkItem1The factor by which the item shrinks if the container size is insufficient.
gapBoth0Defines the gutter between items. Modern browsers support this in Flexbox, not just Grid.

Frequently Asked Questions

In Flexbox, there is no "right" or "left" in the traditional sense. Alignment is based on axes. To align to the end of the container, use "flex-end". Furthermore, verify your Main Axis direction. If "flex-direction" is "column", "justify-content" controls vertical alignment, not horizontal.
Overflow usually occurs when "flex-shrink" is set to 0, or the content inside the item has a fixed width/height that prevents shrinking. Ensure "min-width" is set to 0 (the default is sometimes "auto" which implies content size) and that "flex-shrink" is enabled (1).
The Holy Grail Layout consists of a header, footer, and a main content area flanked by two sidebars. In Flexbox, this is achieved using a column-direction container for the page, and a row-direction nested container for the center section, using "flex-grow: 1" on the middle content to fill available space.
Yes and no. The "gap" property is superior for defining consistent spacing between items because it does not apply to the outer edges of the container (unlike margins which require :last-child hacks). However, margins are still useful for "auto" alignment tricks, like pushing a single item to the far right.
The default value for "align-items" is "stretch". This forces all children to match the height of the tallest item in the row. To preserve the image's natural aspect ratio or height, set "align-self: flex-start" or "center" on the image container.