React Native uses Dots Per Inch (DPI) to measure the sizing and anything displayed on the User Interface (UI). This type of measurement allows an application to look uniform across various screen sizes and pixel densities.
For standard use cases, applications can be developed without having to know the specifics of the user's device (e.g. pixel density) since the UI elements will scale automatically. When it is required, there are APIs available such as `PixelRatio` for finding out the pixel density of the user's device.
For the purposes of this guide, we'll focus on the `get` method, but feel free to check out the [React Native documentation](https://facebook.github.io/react-native/docs/dimensions#docsNav) for details on the other methods.
Let's say you want to cover the whole screen with a background image. We'll use the `ImageBackground` component that React Native provides. You can read more about here: https://facebook.github.io/react-native/docs/images#background-image-via-nesting.
What makes React Native so attractive is the ability to write code once and have it run on multiple devices (iPhone, iPad, Pixel, etc), and the `Dimensions` API makes that really easy.
**Note: There have been some known issues in the past with the Dimensions API, such as it not returning the correct information when a user rotates their device. It's best to make sure you test this on actual devices before deploying an application.**