Bootstrap includes a powerful mobile-first flexbox grid system for building layouts of all shapes and sizes. It's based on a 12 column layout and has multiple tiers, one for each [media query range]({{ site.baseurl }}/layout/overview/#responsive-breakpoints). You can use it with Sass mixins or our predefined classes.
Bootstrap's grid system uses a series of containers, rows, and columns to layout and align content. It's built with flexbox and is fully responsive. Below is an example and an in-depth look at how the grid comes together.
The above example creates three equal-width columns on small, medium, large, and extra large devices using our predefined grid classes. Those columns are centered in the page with the parent `.container`.
- Containers provide a means to center your site's contents. Use `.container` for fixed width or `.container-fluid` for full width.
- Rows are horizontal groups of columns that ensure your columns are lined up properly. We use the negative margin method on `.row` to ensure all your content is aligned properly down the left side.
- Content should be placed within columns, and only columns may be immediate children of rows.
- Thanks to flexbox, grid columns without a set width will automatically layout with equal widths. For example, four instances of `.col-sm` will each automatically be 25% wide for small breakpoints.
- Column classes indicate the number of columns you'd like to use out of the possible 12 per row. So, if you want three equal-width columns, you can use `.col-sm-4`.
- Column `width`s are set in percentages, so they're always fluid and sized relative to their parent element.
- Columns have horizontal `padding` to create the gutters between individual columns, however, you can remove the `margin` from rows and `padding` from columns with `.no-gutters` on the `.row`.
- There are five grid tiers, one for each [responsive breakpoint]({{ site.baseurl }}/layout/overview/#responsive-breakpoints): all breakpoints (extra small), small, medium, large, and extra large.
- Grid tiers are based on minimum widths, meaning they apply to that one tier and all those above it (e.g., `.col-sm-4` applies to small, medium, large, and extra large devices).
- You can use predefined grid classes or Sass mixins for more semantic markup.
Be aware of the limitations and [bugs around flexbox](https://github.com/philipwalton/flexbugs), like the [inability to use some HTML elements as flex containers](https://github.com/philipwalton/flexbugs#9-some-html-elements-cant-be-flex-containers).
Sounds good? Great, let's move on to seeing all that in an example.
While Bootstrap uses `em`s or `rem`s for defining most sizes, `px`s are used for grid breakpoints and container widths. This is because the viewport width is in pixels and does not change with the [font size](https://drafts.csswg.org/mediaqueries-3/#units).
Utilize breakpoint-specific column classes for equal-width columns. Add any number of unit-less classes for each breakpoint you need and every column will be the same width.
Auto-layout for flexbox grid columns also means you can set the width of one column and the others will automatically resize around it. You may use predefined grid classes (as shown below), grid mixins, or inline widths. Note that the other columns will resize no matter the width of the center column.
Using the `col-{breakpoint}-auto` classes, columns can size itself based on the natural width of its content. This is super handy with single line content like inputs, numbers, etc. This, in conjunction with [horizontal alignment](#horizontal-alignment) classes, is very useful for centering layouts with uneven column sizes as viewport width changes.
Create equal-width columns that span multiple rows by inserting a `.w-100` where you want the columns to break to a new line. Make the breaks responsive by mixing the `.w-100` with some [responsive display utilities]({{ site.baseurl }}/utilities/display-property/).
Bootstrap's grid includes five tiers of predefined classes for building complex responsive layouts. Customize the size of your columns on extra small, small, medium, large, or extra large devices however you see fit.
For grids that are the same from the smallest of devices to the largest, use the `.col` and `.col-*` classes. Specify a numbered class when you need a particularly sized column; otherwise, feel free to stick to `.col`.
Using a single set of `.col-sm-*` classes, you can create a basic grid system that starts out stacked on extra small devices before becoming horizontal on desktop (medium) devices.
Don't want your columns to simply stack in some grid tiers? Use a combination of different classes for each tier as needed. See the example below for a better idea of how it all works.
The gutters between columns in our predefined grid classes can be removed with `.no-gutters`. This removes the negative `margin`s from `.row` and the horizontal `padding` from all immediate children columns.
Here's the source code for creating these styles. Note that column overrides are scoped to only the first children columns and are targeted via [attribute selector](https://developer.mozilla.org/en-US/docs/Web/CSS/Attribute_selectors). While this generates a more specific selector, column padding can still be further customized with [spacing utilities]({{ site.baseurl }}/utilities/spacing/).
In practice, here's how it looks. Note you can continue to use this with all other predefined grid classes (including column widths, responsive tiers, reorders, and more).
With the handful of grid tiers available, you're bound to run into issues where, at certain breakpoints, your columns don't clear quite right as one is taller than the other. To fix that, use a combination of a `.clearfix` and our [responsive utility classes]({{ site.baseurl }}/layout/responsive-utilities/).
In addition to column clearing at responsive breakpoints, you may need to **reset offsets, pushes, or pulls**. See this in action in [the grid example]({{ site.baseurl }}/examples/grid/).
Move columns to the right using `.offset-md-*` classes. These classes increase the left margin of a column by `*` columns. For example, `.offset-md-4` moves `.col-md-4` over four columns.
To nest your content with the default grid, add a new `.row` and set of `.col-sm-*` columns within an existing `.col-sm-*` column. Nested rows should include a set of columns that add up to 12 or fewer (it is not required that you use all 12 available columns).
When using Bootstrap's source Sass files, you have the option of using Sass variables and mixins to create custom, semantic, and responsive page layouts. Our [predefined grid classes](#predefined-classes) use these same variables and mixins to provide a whole suite of ready-to-use classes for fast responsive layouts.
Variables and maps determine the number of columns, the gutter width, and the media query point at which to begin floating columns. We use these to generate the predefined grid classes documented above, as well as for the custom mixins listed below.
{% highlight scss %}
$grid-columns: 12;
$grid-gutter-width-base: 30px;
$grid-gutter-widths: (
xs: $grid-gutter-width-base, // 30px
sm: $grid-gutter-width-base, // 30px
md: $grid-gutter-width-base, // 30px
lg: $grid-gutter-width-base, // 30px
xl: $grid-gutter-width-base // 30px
)
$grid-breakpoints: (
// Extra small screen / phone
xs: 0,
// Small screen / phone
sm: 576px,
// Medium screen / tablet
md: 768px,
// Large screen / desktop
lg: 992px,
// Extra large screen / wide desktop
xl: 1200px
);
$container-max-widths: (
sm: 540px,
md: 720px,
lg: 960px,
xl: 1140px
);
{% endhighlight %}
### Mixins
Mixins are used in conjunction with the grid variables to generate semantic CSS for individual grid columns.
{% highlight scss %}
// Creates a wrapper for a series of columns
@mixin make-row($gutters: $grid-gutter-widths) {
display: flex;
flex-wrap: wrap;
@each $breakpoint in map-keys($gutters) {
@include media-breakpoint-up($breakpoint) {
$gutter: map-get($gutters, $breakpoint);
margin-right: ($gutter / -2);
margin-left: ($gutter / -2);
}
}
}
// Make the element grid-ready (applying everything but the width)
You can modify the variables to your own custom values, or just use the mixins with their default values. Here's an example of using the default settings to create a two-column layout with a gap between.
See it in action in <ahref="https://jsbin.com/ruxona/edit?html,output">this rendered example</a>.
Using our built-in grid Sass variables and maps, it's possible to completely customize the predefined grid classes. Change the number of tiers, the media query dimensions, and the container widths—then recompile.
The number of grid columns and their horizontal padding (aka, gutters) can be modified via Sass variables. `$grid-columns` is used to generate the widths (in percent) of each individual column while `$grid-gutter-widths` allows breakpoint-specific widths that are divided evenly across `padding-left` and `padding-right` for the column gutters.
Moving beyond the columns themselves, you may also customize the number of grid tiers. If you wanted just three grid tiers, you'd update the `$grid-breakpoints` and `$container-max-widths` to something like this:
When making any changes to the Sass variables or maps, you'll need to save your changes and recompile. Doing so will out a brand new set of predefined grid classes for column widths, offsets, pushes, and pulls. Responsive visibility utilities will also be updated to use the custom breakpoints.
