description: Use our powerful mobile-first flexbox grid to build layouts of all shapes and sizes thanks to a twelve column system, six default responsive tiers, Sass variables and mixins, and dozens of predefined classes.
Bootstrap's grid system uses a series of containers, rows, and columns to layout and align content. It's built with [flexbox](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Flexible_Box_Layout/Basic_Concepts_of_Flexbox) and is fully responsive. Below is an example and an in-depth explanation for how the grid system comes together.
**New to or unfamiliar with flexbox?** [Read this CSS Tricks flexbox guide](https://css-tricks.com/snippets/css/a-guide-to-flexbox/#flexbox-background) for background, terminology, guidelines, and code snippets.
The above example creates three equal-width columns across all devices and viewports using our predefined grid classes. Those columns are centered in the page with the parent `.container`.
## How it works
Breaking it down, here's how the grid system comes together:
- **Our grid supports [six responsive breakpoints]({{< docsref "/layout/breakpoints" >}}).** Breakpoints are based on `min-width` media queries, meaning they affect that breakpoint and all those above it (e.g., `.col-sm-4` applies to `sm`, `md`, `lg`, `xl`, and `xxl`). This means you can control container and column sizing and behavior by each breakpoint.
- **Containers center and horizontally pad your content.** Use `.container` for a responsive pixel width, `.container-fluid` for `width: 100%` across all viewports and devices, or a responsive container (e.g., `.container-md`) for a combination of fluid and pixel widths.
- **Rows are wrappers for columns.** Each column has horizontal `padding` (called a gutter) for controlling the space between them. This `padding` is then counteracted on the rows with negative margins to ensure the content in your columns is visually aligned down the left side. Rows also support modifier classes to [uniformly apply column sizing](#row-columns) and [gutter classes]({{< docsref "/layout/gutters" >}}) to change the spacing of your content.
- **Columns are incredibly flexible.** There are 12 template columns available per row, allowing you to create different combinations of elements that span any number of columns. Column classes indicate the number of template columns to span (e.g., `col-4` spans four). `width`s are set in percentages so you always have the same relative sizing.
- **Gutters are also responsive and customizable.** [Gutter classes are available]({{< docsref "/layout/gutters" >}}) across all breakpoints, with all the same sizes as our [margin and padding spacing]({{< docsref "/utilities/spacing" >}}). Change horizontal gutters with `.gx-*` classes, vertical gutters with `.gy-*`, or all gutters with `.g-*` classes. `.g-0` is also available to remove gutters.
- **Sass variables, maps, and mixins power the grid.** If you don't want to use the predefined grid classes in Bootstrap, you can use our [grid's source Sass](#sass) to create your own with more semantic markup. We also include some CSS custom properties to consume these Sass variables for even greater flexibility for you.
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#flexbug-9).
As noted above, each of these breakpoints have their own container, unique class prefix, and modifiers. Here's how the grid changes across these breakpoints:
For example, here are two grid layouts that apply to every device and viewport, from `xs` to `xxl`. 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 have the sibling columns 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.
Bootstrap's grid includes six 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 and becomes horizontal at the small breakpoint (`sm`).
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.
Use the responsive `.row-cols-*` classes to quickly set the number of columns that best render your content and layout. Whereas normal `.col-*` classes apply to the individual columns (e.g., `.col-md-4`), the row columns classes are set on the parent `.row` as a shortcut. With `.row-cols-auto` you can give the columns their natural width.
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 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.
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.
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 can be modified via Sass variables. `$grid-columns` is used to generate the widths (in percent) of each individual column while `$grid-gutter-width` sets the width for the column gutters.
Moving beyond the columns themselves, you may also customize the number of grid tiers. If you wanted just four 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 output a brand new set of predefined grid classes for column widths, offsets, and ordering. Responsive visibility utilities will also be updated to use the custom breakpoints. Make sure to set grid values in `px` (not `rem`, `em`, or `%`).
