WebM to MP4

How to use WebM to MP4

1. Choose one or more .webm files.
2. Press Convert to MP4; no settings are needed.
3. Wait for the encode to finish. Long screen recordings take a while because every frame is re-encoded.
4. Download the MP4, or a ZIP of several.

Where WebM files come from and why convert

WebM is the open format Google promotes for the web. It carries VP8, VP9 or AV1 video with Opus or Vorbis audio, and Chrome, Firefox and many screen recorders save to it by default. Browsers play it well, but plenty of desktop editors, presentation software and older iPhones still do not.

Here the VP8 or VP9 stream is decoded and re-encoded with libx264 at CRF 23 in yuv420p, while Opus or Vorbis audio becomes 128 kbps AAC. Faststart is set so the MP4 begins playing quickly. Students recording online classes or demos with a browser extension will find this the quickest way to make a clip shareable.

Limitations

• WebM files saved by browser recorders sometimes lack a duration header, so the progress bar may not move smoothly even though encoding is working.
• Transparency in a WebM (VP9 with alpha) is not preserved in the MP4.
• Long recordings at high resolution can be slow and memory-hungry, especially on phones.

Frequently asked questions

Why won't my WebM open on an iPhone?

Support for WebM on iOS is limited and inconsistent across apps. MP4 with H.264 and AAC is the format iPhones handle natively.

Is quality lost going from WebM to MP4?

A little, as both formats are lossy and the video is re-encoded. At CRF 23 the difference is hard to see for screen and camera footage.

Can I convert several recordings in one go?

Yes. Select them together; they are converted sequentially and offered as a ZIP.

Is the WebM sent anywhere?

No. Your screen recording is converted inside this browser tab and stays private.