Free · no signup · never uploaded
MP3 to MP4 Converter
Turn an audio file into a video you can upload anywhere.
Drop an MP3 here or choose a file
or drop your file here.
MP3 · converted in your browser, never uploaded
Runs in your browser — your file is never uploaded.
Short answer
To convert MP3 to MP4, drop your audio below and download the MP4. Since MP4 needs a picture, the tool pairs your audio with a plain black video track — which is what platforms that only accept video require. It runs entirely in your browser.
Why you’d turn audio into a video
Plenty of platforms only accept video files. YouTube is the obvious one — it’s a common way to publish a podcast episode, a mix, a lecture or a demo track — but the same applies to some social uploaders and course platforms. The audio is what matters; the video track just satisfies the format requirement. Pairing the file with a still picture is the standard, minimal way to do that.
Why the video is black, and why the file stays small
The generated picture is a plain black frame at 1280×720. That’s deliberate: an unchanging image compresses to almost nothing under H.264, since every frame after the first says only “nothing changed”. The result is that the MP4 is barely larger than the MP3 was — nearly all of it is still your audio. The audio itself is re-encoded to AAC at 192 kbps, which is what MP4 expects.
Video formats to MP4 — what happens to your file
| Input format | Usual codecs inside | Conversion | Max file size |
|---|---|---|---|
| MOV (QuickTime) | H.264 + AAC | Rewrap (lossless) | 500 MB |
| MKV (Matroska) | H.264 + AAC | Rewrap (lossless) | 500 MB |
| WebM | VP8/VP9 + Opus | Re-encode to H.264 | 100 MB |
| AVI | MPEG-4/DivX | Re-encode to H.264 | 100 MB |
| GIF | Animated GIF | Re-encode to H.264 | 200 MB |
A rewrap copies the existing video and audio streams into an MP4 container untouched — it is lossless and finishes in seconds. A re-encode decodes and re-compresses, so it takes longer and loses a little quality. Size caps differ because everything runs on one thread in your browser.
How this conversion works
Everything happens locally in your browser with ffmpeg — here is exactly how:
- Load the engine. ffmpeg compiled to WebAssembly loads once from this site — your file is never uploaded.
- Generate a video track. a plain black 1280×720 source is synthesised locally with ffmpeg’s lavfi input, since MP4 needs a video stream to be a valid video file.
- Combine and bound the length. the generated picture is paired with your audio and -shortest stops encoding when the audio ends — without it the synthetic source would run forever.
- Encode and return. video is encoded with libx264 and audio re-encoded to AAC at 192 kbps, then muxed into a downloadable MP4.
Assumptions
- Everything runs client-side; speed depends on your device and the audio’s length.
- The video track is a plain black frame — a custom image isn’t supported.
- Audio is re-encoded to AAC and cannot exceed the quality of the source MP3.
Last reviewed: July 19, 2026
Frequently asked questions
How do I convert an MP3 to MP4?+
Drop your .mp3 in above and press convert. The tool pairs your audio with a black video track and hands back an MP4 — all in your browser, with nothing uploaded.
Are my files uploaded to a server?+
No. Everything runs 100% in your browser using ffmpeg compiled to WebAssembly, so your file never leaves your device. Nothing is uploaded, stored, or sent anywhere — and once the converter has loaded it even works offline.
Can I use my own image instead of black?+
Not in this tool — it generates a plain black frame. That keeps the conversion fast and the file small. A custom cover image would need a full video editor.
Will the MP4 be much bigger than the MP3?+
Barely. A completely static picture compresses to almost nothing, so nearly all of the file is still your audio. Expect only a small increase.
Why is the first conversion slower?+
The first time, your browser downloads the ffmpeg engine (about 31 MB) and starts it up. That happens once — later conversions reuse the loaded engine and are much faster.
Is the audio quality preserved?+
The audio is re-encoded to AAC at 192 kbps, which MP4 requires. That’s a high-quality setting, though as with any lossy re-encode it can’t exceed the quality of the source MP3.
Related tools
Video is processed entirely in your browser with ffmpeg and never uploaded.