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+---
+layout: docs
+title: Ratios
+description: Use generated pseudo elements to make an element maintain the aspect ratio of your choosing. Perfect for responsively handling video or slideshow embeds based on the width of the parent.
+group: helpers
+toc: true
+---
+
+## About
+
+Use the ratio helper to manage the aspect ratios of external content like `<iframe>`s, `<embed>`s, `<video>`s, and `<object>`s. These helpers also can be used on any standard HTML child element (e.g., a `<div>` or `<img>`). Styles are applied from the parent `.ratio` class directly to the child.
+
+Aspect ratios are declared in a Sass map and included in each class via CSS variable, which also allows [custom aspect ratios](#custom-ratios).
+
+{{< callout info >}}
+**Pro-Tip!** You don't need `frameborder="0"` on your `<iframe>`s as we override that for you in [Reboot]({{< docsref "/content/reboot" >}}).
+{{< /callout >}}
+
+## Example
+
+Wrap any embed, like an `<iframe>`, in a parent element with `.ratio` and an aspect ratio class. The immediate child element is automatically sized thanks to our universal selector `.ratio > *`.
+
+{{< example >}}
+<div class="ratio ratio-16x9">
+  <iframe src="https://www.youtube.com/embed/zpOULjyy-n8?rel=0" title="YouTube video" allowfullscreen></iframe>
+</div>
+{{< /example >}}
+
+## Aspect ratios
+
+Aspect ratios can be customized with modifier classes. By default the following ratio classes are provided:
+
+{{< example class="bd-example-ratios" >}}
+<div class="ratio ratio-1x1">
+  <div>1x1</div>
+</div>
+<div class="ratio ratio-4x3">
+  <div>4x3</div>
+</div>
+<div class="ratio ratio-16x9">
+  <div>16x9</div>
+</div>
+<div class="ratio ratio-21x9">
+  <div>21x9</div>
+</div>
+{{< /example >}}
+
+## Custom ratios
+
+Each `.ratio-*` class includes a CSS custom property (or CSS variable) in the selector. You can override this CSS variable to create custom aspect ratios on the fly with some quick math on your part.
+
+For example, to create a 2x1 aspect ratio, set `--bs-aspect-ratio: 50%` on the `.ratio`.
+
+{{< example class="bd-example-ratios" >}}
+<div class="ratio" style="--bs-aspect-ratio: 50%;">
+  <div>2x1</div>
+</div>
+{{< /example >}}
+
+This CSS variable makes it easy to modify the aspect ratio across breakpoints. The following is 4x3 to start, but changes to a custom 2x1 at the medium breakpoint.
+
+```scss
+.ratio-4x3 {
+  @include media-breakpoint-up(md) {
+    --bs-aspect-ratio: 50%; // 2x1
+  }
+}
+```
+
+{{< example class="bd-example-ratios bd-example-ratios-breakpoint" >}}
+<div class="ratio ratio-4x3">
+  <div>4x3, then 2x1</div>
+</div>
+{{< /example >}}
+
+
+## Sass map
+
+Within `_variables.scss`, you can change the aspect ratios you want to use. Here's our default `$ratio-aspect-ratios` map. Modify the map as you like and recompile your Sass to put them to use.
+
+{{< scss-docs name="aspect-ratios" file="scss/_variables.scss" >}}