MP4 to MP3 Converter

Three jobs this tool was built for

Saving a lecture or webinar for the commute. A one hour recording of a screen share is mostly static slides. The video stream is doing nothing useful and often accounts for ninety per cent of the file. Pulling just the audio out at 128 kbps leaves you with about 56 MB instead of several hundred, and it plays in any podcast app.

Getting a clean backing track out of a rehearsal video. Phone footage of a band practice is a poor video but a usable reference recording. Extracting the audio at 320 kbps keeps what the microphone captured and drops the shaky picture.

Rescuing a voice memo saved as a video. Screen recorders and some messaging apps hand back MP4 files even when all you wanted was speech. This turns them back into something you can email, edit or file away.

What the tool does to your video

The page loads FFmpeg built for WebAssembly. The engine core is roughly 32 MB, downloaded on the first run and cached after that. When you press the button it drops the video stream entirely, then encodes the remaining audio through the LAME MP3 encoder at the constant bitrate you selected. The result is previewed in an audio player on the page so you can check it before downloading. Everything runs in your browser, so your files never leave your device.

Accepted containers are MP4, MOV, MKV, WEBM, M4V and AVI. If a video has no audio track at all, the conversion fails with a message saying so rather than handing you a silent file.

Why this is a re-encode, not a copy

Most MP4 files carry AAC audio. AAC and MP3 are different codecs, so there is no way to rename or repackage one into the other. If you only wanted the original audio untouched, FFmpeg on a desktop can copy an AAC track straight into an M4A file in a second or two with no quality loss at all. Asking for MP3 means the AAC has to be decoded back to raw samples and then encoded again by LAME, which takes real work and introduces a second round of lossy compression.

In practice the extra loss is small if you pick a sensible bitrate, and MP3 is far more universally supported than M4A, which is usually why people want it. Just be aware you are trading a little fidelity for compatibility, not extracting a pristine original.

Working out your file size before you start

MP3 size is bitrate multiplied by duration, and nothing else. At 128 kbps you use about 0.94 MB per minute. At 192 kbps it is about 1.4 MB per minute. At 320 kbps it is about 2.4 MB per minute.

So a 12 minute video becomes roughly 11 MB at 128 kbps, 17 MB at 192 kbps, or 29 MB at 320 kbps. A two hour conference recording becomes about 113 MB at 128 kbps or 288 MB at 320 kbps. Notice that the size of the video you started with makes no difference at all. A 4 GB 4K file and a 200 MB phone clip of the same length produce MP3s of the same size.

The page shows the detected duration once you pick a file, so you can do this sum yourself before committing.

Speed, memory and the 500 MB warning

WebAssembly cannot use the processor tricks native FFmpeg relies on, so a browser encode is a good deal slower than a desktop one. Audio-only work is still cheap, and a ten minute clip normally finishes in seconds once the engine is cached. The bottleneck is usually reading a large video into memory, not the encoding itself.

Memory is where things break. The entire video has to be held in the browser tab before FFmpeg can touch it, and tabs are capped far below the machine's total RAM. The page warns above 500 MB for that reason. On a phone the ceiling is lower still, and the tab may simply reload instead of showing an error. If a big file fails, move to a desktop browser, or trim the video down first with Trim Video and extract the audio from the shorter clip.

Common questions

Which bitrate should I choose for speech?

128 kbps is fine for a single voice, an interview or a lecture. Speech has a narrow frequency range and gains almost nothing from a higher bitrate. Save 320 kbps for music.

Will the MP3 sound as good as the video's audio?

Slightly worse, because the video's AAC track is decoded and re-encoded. At 192 or 320 kbps the difference is hard to hear on normal headphones or speakers.

Can I extract audio from a video I found online?

Only if you already have the file on your device and have the right to use it. This tool does not fetch anything from a URL, and you should not use it on material you do not have permission to copy.

My file has multiple audio tracks. Which one do I get?

FFmpeg picks the default track. If a film has commentary or a second language you will get whichever the container flags first, and there is no track selector here.

What if I only want part of the audio?

Cut the video down first, then convert. Trim Video handles the cut, and for pure audio format changes afterwards there is the Audio Converter. If the whole video is simply too large to share, Compress Video is the better tool.