Convert MP4 to MP3

Extract the audio from an MP4 video as an MP3 file, right in your browser. Free and private, with no upload, no size limit and no watermark.

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What libmp3lame does to an already-compressed soundtrack

The audio inside a typical MP4 is AAC, and AAC is a lossy codec. It has already thrown away the parts of the signal its model decided you would not miss. Converting that to MP3 does not decode a pristine master and re-package it; it decodes an approximation and hands it to a second, older lossy encoder that makes its own decisions about what to discard.

Encoder people call this generation loss, and it is the one thing worth understanding before you pick a setting. The damage is not usually dramatic, but it is real and it is permanent, so the sensible plan is to convert once from the best source you have rather than repeatedly from earlier conversions.

LAME, running here as libmp3lame, is the encoder that made MP3 tolerable in the first place. It has been refined for a quarter of a century and at a healthy bitrate it hides the seams well. What it cannot do is invent detail that the AAC pass already removed, which is why a 320 kbps MP3 made from a 96 kbps podcast soundtrack is simply a large file containing 96 kbps of information.

Picking 128, 192 or 320 kbps for the material

192 kbps (recommended) is the default because it covers the widest range of material without much thought. Speech, music, mixed content from a video: at 192 kbps most listeners on most equipment will not identify it in a blind comparison against the MP4’s own soundtrack.

Go to 320 kbps (highest quality) when the source is music you care about and you expect to listen on real headphones, or when the MP3 is going into an editing session and will be re-encoded again later. It is the highest bitrate MP3 defines, and it is the cheapest insurance against a second generation of loss.

Drop to 128 kbps (smaller file) for lectures, interviews, meetings and anything else that is one person talking. Voice occupies a narrow slice of the spectrum and survives aggressive compression gracefully, and the file is small enough to sync to a phone without thinking about it. If you want the reasoning in more depth, choosing a bitrate for voice versus music works through the trade-offs properly.

The video track is discarded, not muted

The command starts with -vn, which tells FFmpeg not to select any video stream at all. Nothing is re-encoded to black, nothing is downscaled, and no time is spent on frames: the picture is simply never read. That is why extracting an hour of audio is far quicker than converting an hour of video, and why the size line the tool shows when it finishes usually reports a very large reduction.

That size line is computed from the real byte counts of your input and your output, not from an estimate, so it is a genuine before-and-after for the file you just processed rather than a marketing figure. What you cannot see from it is how many audio streams your source held, since a multi-track MP4 loses everything except the one stream FFmpeg selected.

When the dropzone rejects a file for not being .mp4

This page accepts .mp4 and nothing else. Drop a .mkv and the shell refuses it with a message naming the rejected file and listing the extensions it wanted, and the file never reaches the converter. That check is on the filename, not on the bytes, so it is fast but easy to fool in both directions.

The fix is usually Video to MP3, which takes MOV, MKV, WEBM, AVI, 3GP and the rest through the same libmp3lame path. There is no benefit to renaming a file to sneak it past the check. If you want the audio only from part of a clip, cut the video first with Trim Video, and if you already have the full MP3 and want a section of it, Cut MP3 takes a start and an end time. The rest of the collection lives under audio tools.

Extracting the MP3 from your MP4

  1. Drop your MP4 into the box above, or click Choose a file.
  2. Leave Audio quality on 192 kbps (recommended) unless the source is music, in which case pick 320 kbps (highest quality).
  3. Click Convert MP4 to MP3 and watch the bar. Everything runs in your tab, so a long recording keeps working even if your connection drops.
  4. Download the MP3. It carries your original filename with the extension swapped, so a lecture named week-4.mp4 comes back as week-4.mp3.

Frequently asked questions

My MP4 has an English and a Spanish audio track. Do I get both?

No. MP3 is a single-stream format, so only one audio track can survive the conversion and FFmpeg's default stream selection decides which one. If the wrong language comes out, reorder the tracks in a container editor first, or convert a separate copy of the file per language.

Does 320 kbps recover quality the MP4 lost?

It cannot. The audio inside an MP4 is almost always already AAC, which is itself lossy, so this is a lossy source being re-compressed into a second lossy format. 320 kbps limits how much extra damage the second pass adds, but nothing that the original encode discarded comes back.

Is the MP3 constant bitrate or variable?

Constant, in effect. The command sets a bitrate with a -b:a flag rather than a quality target with -q:a, so every second of audio is given the same allowance instead of letting LAME spend more on dense passages and less on quiet ones. There is no variable-bitrate option on this page.

Why does the very first conversion pause before anything happens?

The FFmpeg WebAssembly core is roughly 31 MB and is fetched from unpkg the first time a tab needs it, then kept for every later run on that same page. So the initial wait is a download rather than encoding, and a second file converts noticeably sooner.

Can I get just the chorus, or the last ten minutes, instead of the whole file?

Not from this page. There is no start or end control here, so the full length of the audio track is always converted. Cut the video down first, or extract everything and trim the resulting MP3 afterwards.

My file is called .mp4 but the box refuses it. What is going on?

The dropzone checks the filename extension only, and this page accepts .mp4 alone. A file renamed from .mkv or .mov is rejected with a message naming the file itself and listing the extensions this page accepts, and a file whose extension really is .mp4 but whose contents are something else will be accepted and then fail during conversion instead.

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