bootstrap-layout（引导布局）

Bootstrap 5.3 Layout System

Bootstrap's layout system is built on flexbox and provides a powerful 12-column grid for creating responsive layouts. This skill covers containers, the grid system, breakpoints, and layout utilities.

Breakpoints

Bootstrap's responsive design uses six default breakpoints:

Breakpoints apply at the specified width and up (mobile-first).

Containers

Containers are the fundamental building block for layouts.

Container Types

<!-- Fixed-width container (max-width at each breakpoint) -->
<div class="container">...</div>

<!-- Full-width container (100% at all breakpoints) -->
<div class="container-fluid">...</div>

<!-- Responsive containers (100% until specified breakpoint) -->
<div class="container-sm">100% until sm, then fixed</div>
<div class="container-md">100% until md, then fixed</div>
<div class="container-lg">100% until lg, then fixed</div>
<div class="container-xl">100% until xl, then fixed</div>
<div class="container-xxl">100% until xxl, then fixed</div>

Container Max-Widths

Grid System

The grid uses a series of containers, rows, and columns.

Basic Structure

<div class="container">
  <div class="row">
    <div class="col">Column 1</div>
    <div class="col">Column 2</div>
    <div class="col">Column 3</div>
  </div>
</div>

Equal-Width Columns

<div class="row">
  <div class="col">1 of 2</div>
  <div class="col">2 of 2</div>
</div>

<div class="row">
  <div class="col">1 of 3</div>
  <div class="col">2 of 3</div>
  <div class="col">3 of 3</div>
</div>

Specific Column Widths

Use .col-{number} for specific widths (1-12):

<div class="row">
  <div class="col-8">8 columns wide</div>
  <div class="col-4">4 columns wide</div>
</div>

<div class="row">
  <div class="col-3">3 columns</div>
  <div class="col-6">6 columns</div>
  <div class="col-3">3 columns</div>
</div>

Responsive Columns

Combine breakpoint infixes for responsive behavior:

<!-- Stack on mobile, side-by-side on md+ -->
<div class="row">
  <div class="col-12 col-md-6">Left on md+</div>
  <div class="col-12 col-md-6">Right on md+</div>
</div>

<!-- Different widths at different breakpoints -->
<div class="row">
  <div class="col-12 col-sm-6 col-lg-4">Responsive column</div>
</div>

Auto-Layout Columns

<!-- One column width set, others auto -->
<div class="row">
  <div class="col">Auto</div>
  <div class="col-6">6 columns</div>
  <div class="col">Auto</div>
</div>

<!-- Variable width content -->
<div class="row justify-content-center">
  <div class="col-auto">Variable width content</div>
</div>

Row Columns

Control the number of columns per row:

<!-- Always 2 columns per row -->
<div class="row row-cols-2">
  <div class="col">Item 1</div>
  <div class="col">Item 2</div>
  <div class="col">Item 3</div>
  <div class="col">Item 4</div>
</div>

<!-- Responsive: 1 on xs, 2 on sm, 3 on md, 4 on lg -->
<div class="row row-cols-1 row-cols-sm-2 row-cols-md-3 row-cols-lg-4">
  <div class="col">Item</div>
  <!-- More items... -->
</div>

Gutters

Gutters are the gaps between columns. Default is 1.5rem (24px).

Horizontal Gutters (gx-*)

<div class="row gx-5">
  <div class="col">Wide horizontal gutters</div>
  <div class="col">Wide horizontal gutters</div>
</div>

Vertical Gutters (gy-*)

<div class="row gy-4">
  <div class="col-6">Vertical gutter when wrapping</div>
  <div class="col-6">Vertical gutter when wrapping</div>
  <div class="col-6">Vertical gutter when wrapping</div>
</div>

Both Gutters (g-*)

<div class="row g-3">
  <div class="col-6">Equal gutters</div>
  <div class="col-6">Equal gutters</div>
</div>

<!-- No gutters -->
<div class="row g-0">
  <div class="col">No gutters</div>
  <div class="col">No gutters</div>
</div>

Responsive Gutters

<div class="row g-2 g-md-4 g-lg-5">
  <div class="col">Responsive gutters</div>
</div>

Column Alignment

Vertical Alignment

<!-- Align all columns in row -->
<div class="row align-items-start">...</div>
<div class="row align-items-center">...</div>
<div class="row align-items-end">...</div>

<!-- Align individual columns -->
<div class="row">
  <div class="col align-self-start">Top</div>
  <div class="col align-self-center">Middle</div>
  <div class="col align-self-end">Bottom</div>
</div>

Horizontal Alignment

<div class="row justify-content-start">...</div>
<div class="row justify-content-center">...</div>
<div class="row justify-content-end">...</div>
<div class="row justify-content-around">...</div>
<div class="row justify-content-between">...</div>
<div class="row justify-content-evenly">...</div>

Column Ordering

Order Classes

<div class="row">
  <div class="col order-3">First in DOM, last visually</div>
  <div class="col order-2">Second</div>
  <div class="col order-1">Third in DOM, first visually</div>
</div>

<!-- Responsive ordering -->
<div class="row">
  <div class="col order-md-2">Second on md+</div>
  <div class="col order-md-1">First on md+</div>
</div>

Offset Classes

<!-- Offset by columns -->
<div class="row">
  <div class="col-md-4 offset-md-4">Centered column</div>
</div>

<!-- Margin utilities for offsets -->
<div class="row">
  <div class="col-md-4 ms-auto">Pushed right</div>
</div>

Nesting

Columns can be nested:

<div class="row">
  <div class="col-9">
    Level 1
    <div class="row">
      <div class="col-6">Level 2</div>
      <div class="col-6">Level 2</div>
    </div>
  </div>
</div>

Advanced Grid Behaviors

Column Wrapping

When more than 12 columns are placed within a single row, the extra columns wrap onto a new line as one unit:

<div class="row">
  <div class="col-9">.col-9</div>
  <div class="col-4">.col-4 (wraps to new line since 9 + 4 = 13 > 12)</div>
  <div class="col-6">.col-6 (continues on the new line)</div>
</div>

Column Breaks

Force columns to a new line by inserting a full-width element:

<div class="row">
  <div class="col-6">Column 1</div>
  <div class="col-6">Column 2</div>
  <!-- Force next columns to break to new line -->
  <div class="w-100"></div>
  <div class="col-6">Column 3 (on new line)</div>
  <div class="col-6">Column 4 (on new line)</div>
</div>

Apply breaks at specific breakpoints using display utilities:

<div class="row">
  <div class="col-6 col-sm-4">Column 1</div>
  <div class="col-6 col-sm-4">Column 2</div>
  <!-- Force break only at md breakpoint and up -->
  <div class="w-100 d-none d-md-block"></div>
  <div class="col-6 col-sm-4">Column 3</div>
</div>

Standalone Column Classes

Use .col-* classes outside a .row to give elements a specific width. When used outside a row, column padding is omitted:

<!-- Element with 25% width (no row wrapper needed) -->
<div class="col-3 p-3 mb-2">
  .col-3: width of 25%
</div>

<!-- Responsive width -->
<div class="col-sm-9 p-3">
  .col-sm-9: width of 75% above sm breakpoint
</div>

Combine with float utilities for responsive floated images:

<div class="clearfix">
  <img src="..." class="col-md-6 float-md-end mb-3 ms-md-3" alt="...">
  <p>Text wraps around the floated image...</p>
</div>

Handling Gutter Overflow

Large gutters (like .gx-5) can cause horizontal overflow. Two solutions:

Add matching padding to the container:

<div class="container px-4">
  <div class="row gx-5">
    <div class="col">Column with large gutter</div>
    <div class="col">Column with large gutter</div>
  </div>
</div>

Or use an overflow-hidden wrapper:

<div class="container overflow-hidden">
  <div class="row gx-5">
    <div class="col">Column with large gutter</div>
    <div class="col">Column with large gutter</div>
  </div>
</div>

CSS Grid (Alternative)

Note: Bootstrap's CSS Grid system is experimental and opt-in as of v5.1.0. It's disabled by default and requires enabling in your Sass configuration.

Bootstrap 5.3 also supports CSS Grid:

<div class="grid">
  <div class="g-col-6">Half width</div>
  <div class="g-col-6">Half width</div>
</div>

<!-- Custom column count -->
<div class="grid" style="--bs-columns: 3;">
  <div class="g-col-1">1/3</div>
  <div class="g-col-2">2/3</div>
</div>

Z-Index Utilities

Bootstrap provides z-index utility classes for controlling stacking order.

Utility Classes

These low single-digit values (1, 2, 3) are useful for controlling component states like default, hover, and active within the same stacking context.

Component Stacking Order

Bootstrap components use a standardized z-index scale with large gaps to prevent conflicts:

Warning: Avoid customizing individual z-index values. The scale is designed as a cohesive system—if you change one value, you likely need to adjust others to maintain proper layering. Use Sass variables ($zindex-dropdown, $zindex-modal, etc.) to customize these values consistently.

See references/grid-reference.md for position utilities that work with z-index.

Common Layout Patterns

Sidebar Layout

<div class="container-fluid">
  <div class="row">
    <nav class="col-md-3 col-lg-2 d-md-block sidebar">
      Sidebar
    </nav>
    <main class="col-md-9 col-lg-10 ms-sm-auto px-md-4">
      Main content
    </main>
  </div>
</div>

Card Grid

<div class="row row-cols-1 row-cols-md-2 row-cols-lg-3 g-4">
  <div class="col">
    <div class="card h-100">Card 1</div>
  </div>
  <div class="col">
    <div class="card h-100">Card 2</div>
  </div>
  <!-- More cards... -->
</div>

Sass Customization

When compiling Bootstrap from source, you can customize the layout system through Sass variables and mixins.

Key Variables

// Override before importing Bootstrap
$grid-columns: 12;           // Change to 16 or 24 for finer control
$grid-gutter-width: 1.5rem;  // Adjust column gaps
$enable-cssgrid: true;       // Enable CSS Grid classes

Media Query Mixins

// Mobile-first (applies at breakpoint and UP)
@include media-breakpoint-up(md) { ... }

// Desktop-first (applies BELOW breakpoint)
@include media-breakpoint-down(lg) { ... }

// Range (applies BETWEEN two breakpoints)
@include media-breakpoint-between(sm, xl) { ... }

// Single breakpoint only
@include media-breakpoint-only(md) { ... }

Grid Mixins

Create semantic grid layouts without utility classes:

.blog-layout {
  @include make-row();
}

.blog-main {
  @include make-col-ready();
  @include make-col(8);  // 8 of 12 columns
}

.blog-sidebar {
  @include make-col-ready();
  @include make-col(4);  // 4 of 12 columns
}

See references/sass-customization.md for complete variable reference and mixin documentation.

Additional Resources

Reference Files

• references/grid-reference.md - Complete grid class reference
• references/sass-customization.md - Sass variables and mixins for layout customization

Example Files

• examples/responsive-layouts.html - Responsive layout patterns
• examples/sass-customization.scss - Sass customization examples