ReplayGain is a proposed technical standard published by David Robinson in 2001 to measure and normalize the perceived loudness of audio in computer audio formats such as MP3 and Ogg Vorbis. It allows media players to normalize loudness for individual tracks or albums. This avoids the common problem of having to manually adjust volume levels between tracks when playing audio files from albums that have been mastered at different loudness levels.

Although this de facto standard is now formally known as ReplayGain,^[1] it was originally known as Replay Gain and is sometimes abbreviated RG.

ReplayGain is supported in a large number of media software and portable devices.

ReplayGain works by first performing a psychoacoustic analysis of an entire audio track or album to measure peak level and perceived loudness. Equal-loudness contours are used to compensate for frequency effects and statistical analysis is used to accommodate for effects related to time. The difference between the measured perceived loudness and the desired target loudness is calculated; this is considered the ideal replay gain value. Typically, the replay gain and peak level values are then stored as metadata in the audio file. ReplayGain-capable audio players use the replay gain metadata to automatically attenuate or amplify the signal on a per-track or per-album basis such that tracks or albums play at a similar loudness level. The peak level metadata can be used to prevent gain adjustments from inducing clipping in the playback device.^[2]

• Peak amplitude is not a reliable indicator of loudness, so consequently peak normalization does not offer reliable normalization of perceived loudness. RMS normalization is more accurate but does not take into account psychoacoustic aspects of loudness perception.
• With dynamic range compression, volume may be altered on the fly on playback producing a variable-gain normalization, as opposed to the constant gain as rendered by ReplayGain. While dynamic range compression is beneficial in keeping volume constant, it changes the artistic intent of the recording.
• Sound Check is a proprietary Apple Inc. technology similar in function to ReplayGain. It is available in iTunes and on the iPod.^[5]
• Standard measurement algorithms for broadcast loudness monitoring applications have recently been developed by the International Telecommunication Union (ITU-R BS.1770) and the European Broadcasting Union (EBU R128).^[6] This new method has been used to measure loudness in newer ReplayGain utilities such as foobar2000 (since 1.1.6)^{[lower-alpha 1]} and loudgain.^{[lower-alpha 2]}

• Alignment level
• Dialnorm
• EBU R 128
• Loudness war