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Variable bitrate (VBR), or less commonly variable bit rate, is a term used in telecommunications and computing that relates to the bitrate used in sound or video encoding. As opposed to constant bitrate (CBR), VBR files vary the amount of output data per time segment. VBR allows a higher bitrate (and therefore more storage space) to be allocated to the more complex segments of media files while less space is allocated to less complex segments. The average of these rates is calculated to produce an average bitrate for the file that will represent its overall sound quality.

MP3, WMA, Vorbis, and AAC audio files can optionally be encoded in VBR. Variable bit rate encoding is also commonly used on MPEG-2 video.

The advantages of VBR are that it produces a better quality-to-space ratio compared to a CBR file of the same size. The bits available are used more flexibly to encode the music more accurately,^{[citation needed]} with fewer bits used in less demanding passages and more bits used in difficult-to-encode passages.

The disadvantages are that it may take more time to encode, as the process is more complex, and that some hardware might not be compatible with VBR files.

As of December 2006, most newly produced portable music devices and software offer modern support for VBR encoded files. Devices that support only CBR encoded files are becoming increasingly difficult to find.^{[citation needed]}

In the past, many hardware and software players could not decode variable bitrate files properly, partly because the various VBR encoders used were not well developed^{[citation needed]}. This resulted in common use of CBR over VBR for the sake of compatibility.

Support for VBR in AAC and MP3 files is found in most modern digital audio players, including those released by Apple Inc., Creative Technology, and SanDisk. Early VBR algorithms occasionally introduced audible artifacts when encoding monotone or minimal tones (for example audiobooks and acoustic music). These artifacts often mimicked a "digital chirp" during the quiet portions of the song or when there was only speaking. As VBR encoding algorithms have improved, these problems have been resolved in subsequent generations of the VBR standard.^{[citation needed]}

• Constant bitrate