mp3rgain

Lossless MP3/AAC volume adjustment - a modern mp3gain / aacgain replacement written in Rust

mp3rgain adjusts MP3 and AAC volume without re-encoding by modifying the global_gain field in each frame. This preserves audio quality while achieving permanent volume changes.

The only actively maintained tool that performs lossless AAC/M4A bitstream gain adjustment. aacgain has been unmaintained since ~2009 and rarely builds on modern 64-bit systems. mp3rgain is the only practical option today for re-encode-free AAC volume normalization.

Features

• Only tool with lossless AAC bitstream gain: re-encode-free global_gain rewrite for AAC/M4A — a capability previously only available in the long-abandoned aacgain
• Lossless & Reversible: No re-encoding, all changes can be undone (MP3 and AAC)
• ReplayGain: Track and album gain analysis for MP3 and AAC/M4A
• Zero dependencies: Single static binary (no ffmpeg, no mp3gain, no aacgain)
• Cross-platform: macOS, Linux, Windows (x86_64 and ARM64)
• mp3gain / aacgain compatible: Drop-in replacement with identical CLI
• GUI Application: Native desktop app for drag-and-drop workflow

Installation

CLI (mp3rgain)

GUI (mp3rgui)

Binaries for all platforms are also available from GitHub Releases.

Quick Start

# Normalize a single track (ReplayGain)
mp3rgain -r song.mp3

# Normalize an album
mp3rgain -a *.mp3

# Manual gain adjustment (+3.0 dB)
mp3rgain -g 2 song.mp3

# Undo changes
mp3rgain -u song.mp3

# Show file info
mp3rgain song.mp3

Migrating from mp3gain?

Already running mp3gain (or aacgain) in a script, Dockerfile, or CI pipeline? mp3rgain is a drop-in replacement — the CLI flags, the TSV output format, and the APEv2 mp3gain_undo tag are all mp3gain-compatible, so existing parsers (e.g. beets) keep working unchanged. For most setups, migration is a one-line substitution:

sed -i 's/\bmp3gain\b/mp3rgain/g' your_script.sh

See docs/migrating-from-mp3gain.md for the full flag equivalence table, Dockerfile/CI substitution patterns, tag interop notes, and the small set of intentional behaviour differences. Bit-level verification lives in docs/compatibility-report.md.

GUI Application

A native GUI application (mp3rgui) is available for users who prefer a graphical interface.

Features: Drag-and-drop, track/album analysis, one-click gain application, clipping warnings, progress indicators.

Install: See Installation above for Homebrew, Winget, and AUR options. Binaries are also available from GitHub Releases:

• mp3rgui-*-macos-universal.dmg (macOS)
• mp3rgui-*-linux-x86_64.tar.gz / mp3rgui-*-linux-arm64.tar.gz (Linux)
• mp3rgui-*-windows-x86_64.zip / mp3rgui-*-windows-arm64.zip (Windows)
• mp3rgui_*_amd64.deb / mp3rgui_*_arm64.deb (Debian/Ubuntu)

macOS manual download: If you see "mp3rgui cannot be opened" warning, run:

xattr -cr /path/to/mp3rgui.app

This is not needed when installing via Homebrew.

Command-Line Options

Run mp3rgain -h for the full list of options.

ReplayGain analysis runs in parallel by default (std::thread::available_parallelism() worker threads). Use -j 1 or MP3RGAIN_THREADS=1 for the legacy serial path. See docs/perf-parallel.md for the design and real-corpus benchmark numbers.

Documentation

• Migration Guide - Drop-in replacement for mp3gain: flag equivalence, sed/Dockerfile/CI substitution patterns, beets config
• Parallel Performance - -j / --threads design and real-corpus benchmark numbers
• Roadmap - Development plans and upcoming features
• Security - Memory safety and CVE analysis
• Compatibility Report - Verification against original mp3gain
• Technical Comparison - Comparison with similar tools
• Use Cases - Integration examples (beets, headroom, etc.)
• Download Stats - Weekly download trends across all platforms

Why mp3rgain?

The original mp3gain has been unmaintained upstream since ~2015 (though distribution maintainers continue to apply security patches). aacgain, its AAC counterpart, has been unmaintained since ~2009 and is effectively unbuildable on modern 64-bit systems. mp3rgain is a modern, memory-safe replacement written in Rust that covers both.

AAC/M4A is the differentiator. No other actively maintained tool can rewrite AAC global_gain in place. Existing alternatives (rsgain, loudgain, FFmpeg) either only write ReplayGain tags (which non-compliant players ignore) or re-encode the audio (lossy). mp3rgain is the only option that gives you a lossless, reversible, player-agnostic AAC volume adjustment today.

mp3rgain implements the ReplayGain 1.0 algorithm (89 dB reference level) for full compatibility with the original mp3gain / aacgain. Loudness values will differ from EBU R128/LUFS-based tools (foobar2000, loudgain, ffmpeg loudnorm).

Use mp3rgain in Docker / CI

Official multi-arch images (linux/amd64, linux/arm64) are published to GHCR:

ghcr.io/m-igashi/mp3rgain:latest
ghcr.io/m-igashi/mp3rgain:v2          # latest 2.x
ghcr.io/m-igashi/mp3rgain:v2.3.0      # exact version

The image is built FROM scratch with a fully static (musl) binary — no shell, no runtime deps, ~2 MB. Drop-in replacement for mp3gain in containerized batch / cron pipelines (e.g. Plex maintenance windows):

# Normalize a music library by mounting it into the container
docker run --rm \
  -v /path/to/music:/music \
  ghcr.io/m-igashi/mp3rgain:latest -r -R /music

# Run as your own user so written files keep correct ownership
docker run --rm \
  --user "$(id -u):$(id -g)" \
  -v /path/to/music:/music \
  ghcr.io/m-igashi/mp3rgain:latest -r -R /music

Because the entrypoint is the binary itself, all mp3rgain flags work exactly the same as the host CLI (-r, -a, -R, -k, -u, …).

Library Usage

use mp3rgain::{apply_gain, analyze};
use std::path::Path;

let frames = apply_gain(Path::new("song.mp3"), 2)?;  // +3.0 dB
let info = analyze(Path::new("song.mp3"))?;

Contributing

Contributions welcome! See CONTRIBUTING.md.

License

MIT License - see LICENSE.

See Also

• Original mp3gain
• headroom - DJ audio loudness optimizer