Mono ↔ Stereo Converter

How to use

1. Hand it the audio file. Once it decodes, the waveform and its channel count appear, with a player to hear what you started with.
2. Pick a direction under Channels. Each one says what it would do to this file; To mono is chosen when the page loads.
3. Choose an output format. M4A is the default the first time; after that the page remembers your pick.
4. Press Convert.
5. Watch the bar. Cancel sits right beside it if you realise you picked the wrong direction.
6. Switch between the Original and Converted takes to compare, then press Download. The copy keeps your file's name with -mono or -stereo added.

About this tool

Half the recording plays in the left ear and the right one sits silent. Or an upload gets bounced because the platform expects two channels and the file only has one. Both of those are channel-layout problems, and this goes in either direction: stereo to mono, or mono to stereo.

To mono averages every channel into one. A stereo file gets left plus right divided by two; a 5.1 file gets all six divided by six. Averaging rather than adding is why nothing clips, because the peak of the result can never rise above the loudest channel that went in. For speech it is close to a free win, since a voice sits in the middle of the stereo picture anyway and a 16-bit WAV comes out at exactly half the bytes.

To stereo does the mechanical thing and copies the one channel into two identical ones. The file is then stereo by every definition a player cares about, and it sounds precisely as it did before, dead centre. Nothing invents width. The layout is read the moment the file loads, so each direction says what it would do to that file, and a direction that would change nothing, like To stereo on a file that already has two channels, holds Convert back instead of encoding the same audio again.

Four output formats sit under the directions, and the one you pick is remembered. M4A (AAC) and Opus are the small ones. WAV is uncompressed 16-bit and FLAC is lossless compression, both larger, both right when the file is heading back into an editor; WAV is also the one whose size the page can state before you press anything. Sample rate is left alone from start to finish. If the level is uneven as well, run the Audio Normalizer afterwards, and the Audio Converter covers a plain format change on its own.

A four-minute interview, folded to mono

Say a phone recorded a four-minute interview as a 48 kHz stereo WAV called interview.wav. Two channels, 48,000 samples a second, 240 seconds, 16 bits each: 43.9 MB. Load it and To mono reads 2 channels averaged into 1; pick WAV and the strip says 43.9 MB → 22 MB before you press Convert.

The channel math finishes before you notice it; what the bar is showing is the encode. The file that lands is interview-mono.wav, one channel, still 48 kHz, and both ears finally hear the same thing.

Good to know

• Mono to stereo duplicates a channel. The file becomes two-channel and the sound stays exactly where it was, in the middle. There is no spatial information hiding in a mono track to recover.
• To stereo does not fold surround down to a pair. On a file with more than two channels it is held, and To mono is the way down.
• Folding a wide mix down averages the channels, so anything panned hard to one side ends up quieter against the middle. Material that is out of phase between left and right can cancel almost to nothing.
• MP3 is missing from the format list. Bundling an MP3 encoder needs a licence this site has never taken on. M4A fills the compact slot instead.