Convert MP4 (or any audio / video) to MP3
This free MP4 to MP3 converter pulls the audio track out of a video and saves it as an MP3, entirely inside your browser tab. Your file is never uploaded, so it works for private recordings and it works offline once the page has loaded. Pick 128, 192 or 320 kbps, choose stereo or mono, and convert files up to 500 MB.
How to use it
- Select or drop an MP4 file of up to 500 MB into the tool above.
- Choose a bitrate: 128, 192 or 320 kbps.
- Choose stereo or mono output.
- Run the conversion and wait while your device decodes and encodes the audio. Long files take longer, and speed depends on your processor.
- Save the finished MP3 to your device.
How the conversion works
The conversion happens in two stages, and both run as code on your own machine.
Stage 1: decoding with the Web Audio API
An MP4 is a container. The audio inside is usually AAC. The tool reads your file into memory and passes it to the browser’s built-in decodeAudioData() function, part of the Web Audio API. The browser uses its own codecs to turn the compressed audio into raw PCM samples, stored as 32-bit floating-point numbers and resampled to the audio context’s sample rate (typically 44.1 or 48 kHz). The video track is ignored.
Because decoding relies on your browser’s codecs, an MP4 whose audio uses a format your browser cannot play will fail to decode. Copy-protected (DRM) video also cannot be decoded this way.
Stage 2: encoding with LAME via lamejs
The raw samples go to lamejs, a JavaScript port of LAME, the long-running open-source MP3 encoder. The samples are converted from floating point to 16-bit integers, and the encoder packs them into MP3 frames. Each MPEG-1 Layer III frame holds 1,152 samples per channel. The encoder works at a constant bitrate, so the output size is predictable:
| Bitrate | Data per second | Approx. size per minute |
|---|---|---|
| 128 kbps | 16,000 bytes | 0.96 MB |
| 192 kbps | 24,000 bytes | 1.44 MB |
| 320 kbps | 40,000 bytes | 2.4 MB |
The formula is bitrate in bits per second ÷ 8 × duration in seconds. A 4-minute track at 192 kbps comes out at roughly 5.8 MB. These sizes are the same whether you choose stereo or mono. Mono combines both channels into one, so the full bitrate goes to a single channel instead of being shared by two.
Why memory matters
Decoded audio is far larger than the compressed original. One hour of stereo audio at 48 kHz, stored as 32-bit floats, takes about 1.38 GB of RAM (3,600 s × 48,000 × 2 channels × 4 bytes). The 500 MB cap applies to the input file. A very long recording can still strain a phone or an older laptop even when the file is under the limit, because the browser has to hold the whole decoded track in memory.
What this tool does not do
- It does not upload, store or analyse your files on a server.
- It does not output video, and it does not accept files over 500 MB.
- It does not restore quality. AAC to MP3 is a lossy-to-lossy conversion, so 320 kbps preserves what is in the source as closely as possible but cannot add back detail the original encoding removed.
- It does not offer variable bitrate (VBR) or bitrates other than 128, 192 and 320 kbps.
- It cannot convert DRM-protected files or audio formats your browser cannot decode.
FAQ
Is my file really not uploaded?
Yes. Decoding happens through your browser’s Web Audio API and encoding through lamejs running in the page. The audio never leaves your device. You can confirm this by loading the page, disconnecting from the internet, and converting a file.
Which bitrate should I choose?
For speech, podcasts and lectures, 128 kbps is plenty, and 128 kbps mono is a sensible choice for voice-only recordings. For music, 192 kbps is a good balance of size and quality. Choose 320 kbps when you want the closest result to the source and file size is not a concern.
Why did my MP4 fail to convert?
The most common causes are an audio codec your browser cannot decode, DRM protection, or the device running out of memory on a very long file. Trying a different browser can fix codec issues, since browsers ship different decoders. Files over 500 MB are not accepted at all.
Will the MP3 sound worse than the video’s audio?
Slightly, at least in principle, because you are re-encoding audio that was already compressed. At 192 or 320 kbps most listeners will not hear a difference on typical material. At 128 kbps the loss is easier to notice on detailed music, but it is rarely a problem for speech.