Free Mono ↔ Stereo Converter

Worked example

A 3:30 phone-call recording saved as a 44.1 kHz stereo WAV, 35.3 MB, where only the right channel has sound. After the file loads, the inspector reads "Stereo · 44.1 kHz · 3:30" with the Left bar at -90.0 dB, and a note says the left channel is silent and "Right channel to both ears" has been selected. Press "Process audio" and you get a 16-bit stereo WAV of 35.3 MB with the same sound in both ears. Pick "Stereo → Mono (mix down)" instead and you get a 17.7 MB mono file; because one side is silent, the tool keeps the live channel at full level instead of averaging it with silence, which would be 6 dB quieter. Choose "MP3" and the 192 kbps result is about 4.8 MB.

Which settings to use, and the limits

• Read the channel inspector before choosing. After you add a file the tool shows the channel count, sample rate, length and the level of each channel in dB. If one side of a stereo file is below -60 dB it counts as silent, and the tool preselects the fix. A mono file preselects "Mono → Stereo (duplicate the louder channel)". The other choices are "Left channel to both ears", "Right channel to both ears", "Swap left & right channels" and "Split into separate left + right files".
• Know what the mix down does. "Stereo → Mono (mix down)" averages the two channels, (left + right) / 2. If one channel is silent it keeps the other at full level. If a mono result sounds thin or hollow, the two sides may be partly cancelling each other: tick "Invert right channel phase (fixes thin or cancelling mono mixes)", or use "Left channel to both ears" to keep just one side.
• Balance the sides with the volume sliders. "Left volume" and "Right volume" run from -12 to +12 dB in 1 dB steps and are applied before the operation, so they also change the mix down and the split files. They only appear for a file with two channels. A file with more than two channels is read as its first two channels.
• Choose the output format and bit depth. WAV is the default and is instant, with a "WAV bit depth" of 16-bit, 24-bit or 32-bit float. It keeps the file's own sample rate and is about 10 MB per minute for 16-bit stereo at 44.1 kHz, and half that for mono. "MP3" and "M4A" are encoded at 192 kbps and are much smaller, but they need a one-time encoder download of about 30 MB.
• Split gives both channels in one run. "Split into separate left + right files" makes both mono files at once, named like "song_left.wav" and "song_right.wav", with "Download left channel", "Download right channel" and "Download both (ZIP)". For the other operations the file is named like "song_stereo_to_mono.wav", and "Keep the original file name (no suffix when one file is produced)" removes the suffix.
• Know the file limits. The tool takes one file at a time, audio or video, and decodes the whole file in memory. A file above about 2 GB is refused on a normal computer, above about 1 GB on a computer reporting 4 GB of memory or less, and above about 400 MB on a phone. Video files and audio the browser cannot decode itself are read with the media encoder instead, which is the same one-time download of about 30 MB. Your file is processed in the browser and is not uploaded.

Common problems and how to fix them

It says "Could not process this audio file. It may be an unsupported or corrupted format."

Neither the browser nor the media encoder could read the file. Try the original recording, or save it as WAV or MP3 and add that. A video with no sound track cannot be read either. For a video or an unusual format the tool needs the encoder download, so this message can also appear if that download fails while you are offline.

It says "This file is 2.6 GB, which is over the ~2 GB this device can process in the browser. Try a smaller file or trim a shorter clip first."

The sizes in the message are your file and the limit for this device, about 2 GB, 1 GB or 400 MB. Cut a shorter piece with an audio trimmer first, or try a desktop computer, which allows larger files than a phone.

Splitting gave me only one file.

The file has only one channel, so there is nothing to split into a second file. The inspector shows "Mono" for such a file. Use "Mono → Stereo (duplicate the louder channel)" if you need a two-channel file.

It says "Failed to load the encoder. Check your internet connection and try again."

You picked MP3 or M4A, which need the encoder of about 30 MB, and it could not be downloaded. Check your connection and press "Process audio" again, or switch to WAV, which is made in the browser without the encoder.

It says "Canceled." and there is no file.

That follows pressing "Cancel" during the MP3 or M4A encode. Press "Process audio" again; the encoder restarts. The cancel button only appears while MP3 or M4A is being encoded.

How it compares

This tool does a few whole-file channel jobs and nothing else. The mix down is a plain average of left and right, so it has no pan law or stereo width choice, and "Left channel to both ears" is the alternative when you want one side exactly as recorded. It works on the first two channels only, with no per-region editing. A multitrack audio editor handles those jobs, plus surround layouts and selective edits, but needs installing. Here you open the file, pick one of six operations and download the result.