WEBM to MP4
Re-encodes your WEBM to H.264 video and AAC audio inside an MP4 container, in the browser.
Drop a WEBM video here
Everything happens on your device — your files are never uploaded.
Done
What this does
This repacks a WEBM video into an MP4 container. WEBM holds VP9 (or VP8) video and Opus audio; MP4 needs H.264 video and AAC audio, so the streams are decoded and re-encoded to those codecs rather than copied. Re-encoding video is lossy: H.264 starts from the already-compressed VP9 frames, so the result is a generation older than the source even when it looks identical.
Frame size, frame rate, and duration are preserved. The audio track is re-encoded to AAC; the picture is moved to a faststart MP4 so it can begin playing before the whole file downloads. The conversion runs in your browser through WebCodecs, so the file is read locally and never uploaded.
How it works
- 1 Drop your WEBM video or pick it from your device.
- 2 Click Convert to MP4 and watch the progress bar.
- 3 Download the MP4.
Built with open source
- Mediabunny — Converts and edits video and audio in the browser via WebCodecs. Add-on encoders cover MP3, AAC, and FLAC. · MPL-2.0
Frequently asked
Is my video uploaded anywhere? +
No. The conversion runs in your browser with WebCodecs. Your WEBM file never leaves your device, there's no watermark or sign-up, and the only size cap is your computer's memory.
Does converting lose quality? +
Yes, slightly. WEBM's VP9 frames are decoded and re-encoded as H.264, so the MP4 is a second lossy generation. It usually looks the same, but it isn't a bit-for-bit copy of the original.
Why convert WEBM to MP4? +
WEBM uses VP9/Opus, which many apps and devices reject. MP4 uses H.264 video and AAC audio, which play natively on phones, Macs, Windows, TVs, and editors like Premiere and iMovie.
It says my browser can't encode H.264. What now? +
MP4 needs an H.264 encoder. Chrome and Edge include one, Firefox currently does not. Open this page in Chrome or Edge and try again.
What happens to the audio? +
Opus audio is re-encoded to AAC, the audio codec MP4 expects. If your WEBM has no audio track, the MP4 comes out silent too.
Can I convert several files at once? +
This page handles one video at a time. There's no hard file limit across runs, but a single very large file is bounded by your device's memory, since the whole video is processed in the browser.
Will the file get smaller? +
Not reliably. Output size depends on the H.264 and AAC encoders, not on a size target. Some clips shrink, others grow; this tool changes the format, it doesn't compress to a chosen size.