All about CAF files

Apple's flexible audio container with no size limit

Last updated on April 25, 2026 by Sören Ramspeck with the experience of more than 128,393,575 converted files since 2013.

The Core Audio Format (CAF) is a container format developed by Apple for storing and transferring digital audio data. Apple introduced the format in 2005 with Mac OS X 10.4 (Tiger) to overcome the technical limitations of some older formats such as WAV and AIFF. CAF joins a long list of Apple’s own audio formats, including AIFF, QTA, and M4A.

No size limit

Unlike WAV or AIFF files, which are limited to a maximum file size of 4 GB (which, for these uncompressed formats at 24-bit/48 kHz, corresponds to only about six hours of recording time), the CAF format uses 64-bit file offsets. This effectively eliminates this size limitation.

CAF is a container format

CAF is not a codec, but a pure container. This means that various audio codecs can be used within a CAF file, including uncompressed linear PCM (in various bit depths and sample rates), Apple Lossless (ALAC), AAC, IMA 4:1 ADPCM, and MP3. The quality of the CAF file—whether lossless or lossy—depends, as usual, on the codec used.

Current Uses of CAF Files

As is typical for container formats, there is a diverse mix of codecs in use. The most common current use case is longer voice messages in iMessage, which used to be .AMR files. The following table provides an (incomplete) overview of current CAF usage:

Resilient to recording interruptions

Another advantage of CAF over WAV and AIFF lies in its reliable recording behavior: While the header of WAV files must be finalized at the end of the recording (which can result in an unusable file if the system crashes), CAF allows new audio data to be appended without having to complete the header in advance. The content can be reconstructed even without a finalized header.

Technical Structure

The format supports any number of audio channels and is therefore also suitable for multichannel and surround sound productions. In addition to the RAW audio data, a CAF file can store various metadata, including text annotations, markers, region information, channel layouts, instrument information, and peak data for the waveform display.
Every CAF file begins with a file header containing the file type (the ASCII string “caff”) and the format version. This is followed by an Audio Description Chunk, which describes the audio format, and an Audio Data Chunk containing the actual audio data. Variable bit rates additionally require a Packet Table Chunk. Files with more than two channels require a Channel Layout Chunk.

Summary

In practice, CAF is primarily used in Apple development for iOS and macOS applications. Apple’s DAWs Logic Pro and GarageBand use CAF internally, particularly for loop and sound effect libraries. Xcode also uses CAF as the preferred format for audio assets in app projects. Outside the Apple ecosystem, the format has hardly gained traction; WAV, FLAC, and M4A dominate cross-platform use.

Conversion

Since CAF is sparsely supported outside the Apple ecosystem, converting it to more widely used formats before sharing makes sense. file-converter-online.com detects the codec used in the CAF file and automatically performs the appropriate conversion. Generally, conversion to MP3 will offer good quality with an acceptable file size. For lossless conversion, FLAC is the ideal target format. If file size isn’t a concern, WAV offers the best compatibility among lossless formats.

Sources

[1] MacRumors: Apple Improves Audio Messages Quality in iOS 12.2
[2] Apple: Multimedia Programming Guide – Using Audio
[3] Wiki: Core Audio Format
[4] Supported Audio File and Data Formats in OS X