Free Lossless Audio Codec (FLAC) is a file format for lossless audio data compression. During compression, FLAC does not lose quality from the audio stream, as lossy compression formats such as MP3, AAC, and Vorbis do. Josh Coalson is the primary author of FLAC.

FLAC reduces bandwidth and storage requirements without sacrificing the integrity of the audio source. A digital audio recording (such as a CD track) encoded to FLAC can be decompressed into an identical copy of the audio data. Audio sources encoded to FLAC are typically reduced in size 40 to 50 percent (46% according to their own comparison).

FLAC is suitable for everyday audio playback and archival, with support for tagging, cover art and fast seeking. FLAC's free and open source royalty-free nature makes it well-supported by many software applications, but FLAC playback support in portable audio devices and dedicated audio systems is limited at this time.

On January 29, 2003, Xiphophorus (now called the Xiph.Org Foundation) announced the incorporation of FLAC under their banner, alongside Vorbis, Theora, Speex, and others.

The project

The FLAC project consists of:

• The stream formats
• A simple container format for the stream, also called FLAC (or Native FLAC)
• libFLAC, a library of reference encoders and decoders, and a metadata interface
• libFLAC++, an object wrapper around libFLAC
• flac, a command-line program based on libFLAC to encode and decode FLAC streams
• metaflac, a command-line metadata editor for .flac files and for applying Replay Gain
• Input plugins for various music players (Winamp, XMMS, foobar2000, musikCube, and many more)
• With Xiph.org incorporation, the Ogg container format, suitable for streaming (also called Ogg FLAC)

"Free" means that the specification of the stream format can be implemented by anyone without prior permission (Xiph.org reserves the right to set the FLAC specification and certify compliance), and that neither the FLAC format nor any of the implemented encoding/decoding methods are covered by any patent. It also means that the reference implementation is free software. The sources for libFLAC and libFLAC++ are available under Xiph.org's BSD license, and the sources for flac, metaflac, and the plugins are available under the GPL.

In its stated goals, the FLAC project encourages its developers not to implement copy prevention features of any kind.

Comparisons

FLAC is specifically designed for efficient packing of audio data, unlike general lossless algorithms such as DEFLATE which is used in ZIP and gzip. While ZIP may compress a CD-quality audio file by 10–20%, FLAC achieves compression rates of 30–50% for most music, with significantly greater compression for voice recordings. By contrast, lossy codecs can achieve ratios of 80% or more by discarding data from the original stream.