Convert stereo audio to mono in your browser: mix both channels or keep only the left or right, with a phase check first. Free, no upload.
The stereo to mono converter turns a two-channel file into a single channel. Mix left + right is the standard conversion: the two sides are averaged, so everything in the recording is kept. Left only and Right only take one side and discard the other, which is what you want when an interview or instrument was recorded on one channel and the other is empty or noisy. Before you export, the tool compares the two channels and warns you if they are out of phase, because mixing those would cancel much of the sound.
Mono is the right choice for a single voice, for audio headed to a phone line or a PA system, and for halving the size of an uncompressed WAV. The cost is the stereo image: anything that was placed left or right ends up in the middle, and very wide effects can come out quieter. Your audio never leaves your device. Mono vs stereo audio explains when each makes sense, and what is phase cancellation covers why some sounds disappear in a mix. To keep both sides as separate files, use the channel splitter.
Mix both for music and for anything recorded in normal stereo, so that nothing is lost. Keep just the left or right when only one side has the sound you want, such as a microphone recorded onto a single channel, or when the tool warns that the two sides are out of phase.
When the left and right channels contain the same sound with opposite polarity, adding them together cancels it. This is called phase cancellation. It affects some stereo-widened effects and recordings made with a wiring or microphone fault. Choosing one side instead of mixing avoids it.
An uncompressed WAV is half the size in mono, because it stores one channel instead of two. For MP3 and Opus the file size depends on the bitrate you choose rather than the channel count, so a mono file at the same bitrate is the same size, though it will sound better at low bitrates.
When a file loads, the tool compares the left and right channels across the whole recording and scores how alike they are, from +1 for identical through 0 for unrelated to -1 for opposite polarity. Below -0.2 it warns that mixing will cancel much of the sound and suggests keeping one side. Above 0.98 it notes that the two sides are nearly identical, so all three options will sound the same.
Sometimes, slightly. Mix left + right averages the two sides, so anything identical in both, such as a centered vocal or bass, keeps its level, while sound that differs between them adds up only partly and comes out lower. Wide reverbs, room ambience, and hard-panned instruments are the parts most likely to drop. Averaging can never push peaks above the original, so the mix will not clip, and Left only or Right only copies one side at its original level.