MP3Gain — The complete guide to consistent volume.

Version 6.5.0 — Professional loudness normalization, from your first file to your entire library.

1. What Does MP3Gain Do?

Have you ever made a playlist where some songs are too quiet and others blast your ears? MP3Gain fixes that.

It analyzes each audio file, measures its loudness, and adjusts it to a consistent level. After processing, all your tracks play at roughly the same volume — no more reaching for the volume knob between songs.

MP3Gain works with MP3, AAC, FLAC, WAV, and most other audio formats.

Example: A classical recording at -30 dB and a pop song at -12 dB can both be normalized to the same target level, so they sound equally loud.

2. Quick Start

Click Add to choose audio files from your computer.
(Optional) Click the gear icon to adjust loudness settings.
Select the files and click the Start button.
Find your processed files in the output folder — or in the original location if you replaced them.

That's it! The free edition lets you adjust up to 3 files per day.

3. Adding Your Music

There are several ways to add files:

Add button — Opens a file browser. Pick one or more audio files.
Dropdown menu — Next to the Add button. Options include Music Library (import from your Music app) and Recent (quickly re-import files you've worked on before).
Drag & drop — Drag audio files from Finder right onto the list.

Once added, each file shows its filename, current volume level, and the target you've set.

Tip: The right arrow (→) between input and output info shows the direction of processing — from original to processed.

4. Choosing a Method

MP3Gain offers three ways to adjust volume. The right choice depends on your goal:

Loudness (Recommended)

Uses the EBU R128 broadcast standard. This method measures how loud a track sounds to human ears, not just the electrical signal. It's the best choice for playlists and albums because tracks will feel equally loud even if they're very different types of music.

Best for: playlists, albums, podcasts, video soundtracks.

Volume

Adjusts the average signal level (RMS) to a specific decibel value. A straightforward approach — you set a target number and MP3Gain moves everything to that level. Less sophisticated than Loudness mode but faster and simpler.

Best for: quick adjustments when you know your target dB level.

Peak Limit

Prevents clipping — the harsh distortion that happens when a signal exceeds the maximum digital level. This mode ensures no part of the audio goes above your set limit. It doesn't make quiet parts louder, it only caps the loudest peaks.

Best for: fixing recordings that distort, or setting a safety ceiling before uploading to streaming platforms.

5. Fine-Tuning Settings

Click the gear icon next to the target display to open the configuration panel:

Using Loudness Mode

Three sliders let you fine-tune the result:

Integrated Loudness — The overall loudness target. Lower numbers (more negative) = quieter. Higher (closer to 0) = louder. Streaming platforms typically use around -14 to -24 LUFS.
Loudness Range — How much the volume can vary within a track. Lower = more consistent. Higher = more dynamic range preserved.
True Peak — The absolute maximum level. Setting this to -1 or -2 dB leaves a small safety margin to prevent distortion on playback devices.

Using Volume or Peak Limit Mode

A single slider controls the target level in decibels (dB). Volume mode sets the average; Peak Limit mode sets the maximum.

You can type exact numbers into the value fields next to each slider for precision.

What are LUFS? LUFS stands for "Loudness Units relative to Full Scale." It's the standard way to measure how loud audio sounds to human ears. You don't need to understand the science — just know that two files at the same LUFS value will sound equally loud.

6. Processing & Output

Processing Your Files

Select the files you want to process in the list, then use the action button in the toolbar:

Button	Meaning
Start	Selected files are ready. Click to begin processing.
Stop	Files are currently processing. Click to cancel.
Grayed out	No files are ready to process (they're already done or currently running).

Understanding File Status

Each file in the list shows an icon indicating its current state:

Ready	Waiting to be processed.
Queued	Waiting for an available slot. Concurrent processing scales with your CPU, up to 4 files at once.
Processing	The progress ring fills up as the file is processed.
Success (green)	Click to reveal the output file in Finder.
Failed (red)	Something went wrong. Click to open the log for details.

Where Do the Files Go?

By default, processed files are saved to a folder you choose. Click the folder icon to select an output location.

If you check "Replace original file": the processed version overwrites the original file in place. Use with caution — the original is permanently replaced. When this option is on, the folder selector is disabled since no extra folder is needed.

Important: Once you replace the original file, you cannot undo the change. If you want to keep the original, uncheck this option and save to a separate folder.

7. Saving & Reusing Settings

If you frequently use the same loudness settings, save them as a preset. Presets appear in the dropdown at the bottom of the configuration panel:

Built-in presets: Default, Podcast, Music, Streaming-video — ready to use.
Save a preset: Adjust settings the way you like, click +, give it a name.
Delete a preset: Select your custom preset and click –. Built-in presets can't be deleted.

Selecting a preset instantly applies its settings. Your custom presets are saved securely on your device.

8. Keyboard Shortcuts

`⌘ O`	Open file dialog
`⌘ L`	Show processing log
`⌫ Delete`	Remove selected files (only removes completed/ready files)
`Esc`	Close configuration panel and other dialogs
`↩ Return`	Confirm settings (OK button in panels)

9. Help & Troubleshooting

Processing fails: Click the red indicator on the failed file to view the log. Common causes:

File is corrupted or uses an unsupported format.
The output folder is no longer accessible (e.g., external drive disconnected).
Not enough disk space for the processed file.

Files sound the same after processing: If the original volume is already close to your target, the change may be subtle. Check the log for actual measured values.

Progress bar jumps to 100% instantly: Very short audio files process quickly. This is normal.

Window won't close: If files are still processing, MP3Gain will ask if you want to stop them before closing. Confirm to cancel all processing and close.

10. Glossary of Terms

Clipping: Harsh distortion that happens when audio exceeds the maximum digital level. Sounds like crackling or buzzing.
dB (Decibel): A unit for measuring sound level. In digital audio, 0 dB is the maximum, and negative numbers indicate how far below maximum. -20 dB is quieter than -10 dB.
EBU R128: A European broadcast standard for measuring loudness. It's designed to match how human ears perceive volume, not just the raw electrical signal. Used by TV, radio, and streaming services.
LUFS: "Loudness Units relative to Full Scale." The standard unit for perceived loudness. Streaming platforms often target around -14 to -16 LUFS. Lower (more negative) = quieter.
Loudness Range (LRA): How much the volume varies within a track. A classical piece might have wide range (quiet passages to loud crescendos); a pop song typically has narrow range.
Normalization: The process of adjusting audio so that multiple files play at a consistent volume level.
Peak: The single loudest moment in an audio file. Keeping peaks below 0 dB prevents distortion.
RMS (Root Mean Square): A mathematical way to measure the average level of an audio signal. A simpler, faster measurement than loudness, but less accurate to human perception.
True Peak: A more precise measurement of the absolute maximum level that accounts for what happens between digital samples. Important for preventing distortion on real playback devices.