Table of Contents
Introduction
In Android development you package your app before distributing it to testers or users. For many years this package was almost always an APK file. Today, especially for Google Play, the preferred format is AAB. Understanding the difference between APK and AAB is essential because it affects how you build, test, upload, and deliver your app to users.
This chapter focuses on what is unique to APK and AAB formats, how they are used, and why modern Android development leans toward Android App Bundles.
What is an APK
An APK, short for Android Package, is the installable file format that Android devices use to install applications. It is similar to a .zip archive that contains your app code, compiled resources, and the manifest, all combined into one complete package.
From a developer perspective, when you build an APK, you are producing a single binary that contains everything your app might need. That includes all supported device configurations, languages, and screen densities. When a user installs the APK, the device receives this entire package.
You can install an APK directly on a device, either from Android Studio, via command line, or by sharing the APK file. This makes APKs straightforward for manual distribution, side loading, and direct testing.
What is an AAB
An AAB, short for Android App Bundle, is a publishing format introduced by Google to optimize how apps are delivered to different devices. Instead of creating one full package that contains everything, the App Bundle contains all your app’s compiled code and resources but organizes them so that Google Play can generate smaller, device specific APKs.
You cannot directly install an AAB on a physical device. An AAB is designed to be uploaded to a distribution service, most commonly Google Play. The store then processes the bundle and creates optimized APKs for users based on their devices.
For the developer, this means that the final APK that runs on a user’s device may not be exactly the same file that you built locally. Instead, it is generated from your bundle according to the device configuration.
Main Differences Between APK and AAB
The key difference between APK and AAB is their purpose. An APK is an installable package. An AAB is a publishing bundle used to generate APKs.
With APKs, you package everything into one universal file. With AABs, you upload a bundle and let the distribution platform decide how to split and serve the content. This leads to differences in size, installation, and workflow.
A single universal APK typically contains resources for all screen densities, all supported CPU architectures, and all app languages. As a result, the file can be large, and users download many resources they will never use.
An AAB, on the other hand, is used by Google Play to create configuration specific APKs. For example, Google Play can generate a smaller APK that contains only one CPU architecture, only the screen density needed for the device, and only the languages that the user’s device is configured for. This reduces the size of the download and the installed app.
Important rule: APK is the format installed on devices. AAB is the format uploaded for publishing, which the store uses to generate device specific APKs.
Size and Optimization
One of the main reasons AAB exists is size optimization. Smaller downloads lead to faster installations and less data usage. This is especially important for users with limited storage or slower connections.
With a universal APK, the app contains multiple variants of resources. For example, images may exist in several density folders, and native libraries may exist for several CPU architectures. The device will use only the resources that match its configuration, but all of them are still present in the installed package.
With an AAB based workflow using Google Play, the store analyzes the bundle and creates split APKs that match the user’s device. For a device that uses a specific CPU architecture and a specific language, the user receives only those parts. The result is a smaller app size, both for download and storage.
From the developer perspective, you do not usually manage these splits manually when using AAB with Google Play. The bundle describes all configurations, and the store handles the optimization.
Use Cases for APK
APK files still have important roles in development and distribution. During development, you usually build APKs to test your app on emulators and physical devices. Android Studio installs the APK directly on the device when you click Run or Debug.
For internal testing, QA, and sharing builds outside Google Play, APKs are convenient. You can create a debug or release APK and distribute it through other channels, such as email, internal download pages, or third party distribution services that accept APK uploads.
APK is also required for some app stores or distribution environments that do not support AAB. In those cases, you build a signed release APK and upload it directly.
When performing side loading, where a user manually installs an app from outside an official store, the user needs an APK. They cannot install an AAB file directly.
Use Cases for AAB
AAB is primarily used for publishing to Google Play. Google requires new apps on the Play Store to be published as App Bundles for most typical scenarios. When you upload an AAB, Play generates signed APKs on your behalf and delivers optimized versions to users.
AAB is also useful for testing the final, Play like behavior before public release. Google Play supports internal, closed, and open testing tracks that all accept AAB uploads. This lets testers receive the same optimized APKs users will see, including dynamic features or other app bundle specific behavior.
In addition, the AAB format supports advanced distribution options, such as dynamic feature modules, where parts of the app can be downloaded on demand. While this chapter does not explore those features in detail, they are part of the reason AAB exists and is required on the Play Store.
Building APK and AAB in Android Studio
In Android Studio, you can build both APK and AAB from the same project. During everyday development, when you click Run, Android Studio builds and installs an APK automatically. You usually do not see the AAB in that workflow.
For distribution and publishing, you use the build tools to create a signed artifact. You can generate a signed APK for direct distribution or for other app stores that require APK uploads. You can also generate a signed App Bundle, which is what you upload to Google Play.
From your point of view, the choice is usually simple. Use APK when you need a file that can be installed directly on a device or uploaded to a store that requires APK. Use AAB when publishing to Google Play in order to benefit from automatic optimization and comply with store requirements.
Signed Artifacts and Installation
Although this chapter does not cover signing in depth, it is important to understand how signing relates differently to APK and AAB.
A signed APK is both the package and the signed artifact that installs on the device. The signature is embedded inside the APK, and the device verifies it before installation.
With AAB on Google Play, you sign the bundle, then Google uses that bundle and, depending on your Play signing configuration, may manage the signing keys and sign the device specific APKs that are delivered to users. Users never see the AAB itself. They only receive the generated APKs.
For local testing, you cannot install an AAB directly, so you must work with APKs. The ability to install an APK directly on a device is one of the main practical differences that you will notice as a beginner.
Choosing Between APK and AAB
In practice, you rarely choose between APK and AAB for the same purpose. Instead, you select the format based on what you are trying to do.
For running and debugging on devices, use APK. Android Studio handles this automatically. For manual sharing, side loading, or stores that do not support AAB, generate a signed APK.
For publishing on Google Play, use AAB. The Play Console expects an App Bundle for new apps, and this allows users to receive smaller, device tailored APKs without extra work on your part.
Understanding this separation of roles avoids confusion. APK is what devices install. AAB is what you upload to Google Play so that it can create optimized APKs.
Summary
APK and AAB are closely related but serve different purposes in Android development. APK is the installable package that runs on devices and is used extensively during development, testing, and in some distribution channels. AAB is a publishing format designed for Google Play, where it enables automatic creation of smaller, device specific APKs.
As you build real apps, you will work with both formats. You will test and debug using APK files, and when you are ready to publish on Google Play, you will generate and upload an AAB. Understanding this distinction prepares you for the signing configurations and release builds that follow in the next chapters.