LOCAL VIDEO TOOLS · BROWSER VIDEO OUT
WebM to MP4 Converter
A WebM downloads fine and then refuses to open in QuickTime, Premiere or a phone gallery. This page rewrites it as H.264 MP4 in your browser.
Choose a file. It stays on your device.
How to convert a WebM file to MP4
- 1
Drop the WebM you downloaded or exported. The file is parsed locally.
- 2
Start the conversion. The first video track is re-encoded to H.264 and the first audio track to AAC.
- 3
Play the MP4 in the app that rejected the WebM before you delete the original.
WebM is a delivery format, not an editing format
WebM exists so a browser can play video without a licence fee. It usually carries VP8 or VP9 picture and Vorbis or Opus sound, wrapped in a Matroska-derived container. Sites that serve it are optimising for a web player, not for the file you keep afterwards.
That is why a downloaded WebM often has no camera metadata, no chapter markers and no subtitle track that survives a re-encode. It was built to be streamed, and the copy on your disk is a side effect.
What actually refuses to open it
- QuickTime Player on macOS: no WebM support, so double-clicking gives you nothing.
- Premiere Pro and older After Effects versions: import fails or needs a third-party decoder.
- PowerPoint and Word on Windows: the insert-video dialog filters it out.
- iPhone and iPad Photos: no import, so it never reaches the camera roll.
- Smart TVs and car head units: USB playback is usually MP4 only.
An MP4 with H.264 video and AAC audio is the combination those players were tested against. That is the whole point of this page: not a better file, a file that opens.
The MP4 can be larger than the WebM
VP9 is roughly a third more efficient than H.264 at the same visual quality, and AV1 more so. Re-encoding a lean WebM to H.264 therefore often grows the file, sometimes by half. A 4 MB WebM can turn into a 6 MB MP4 with no visible improvement.
If size is the reason you are here, compress the MP4 afterwards, or lower the resolution first. The conversion itself is a compatibility fix, and compatibility costs bits.
What survives the trip
The first video track becomes H.264 at the source resolution, with the frame size rounded to even numbers. The first audio track becomes AAC at 128 kbps. Opus is decoded and re-encoded, so the result plays on hardware that has never heard of Opus.
Subtitles, extra audio tracks and chapters are dropped. Alpha transparency in a VP8 or VP9 web video becomes a flat background, because H.264 in this profile has no alpha channel.
FAQ
Why will QuickTime not open my WebM?
QuickTime Player only handles the containers Apple ships: MP4 and MOV. WebM is a different container, so the file needs rewriting before it opens, regardless of the codec inside.
Does converting change the quality?
Yes. The picture is decoded and encoded again as H.264, so it is not a lossless copy. Keep the WebM if you may need the original.
Can the MP4 end up bigger?
Often, yes. VP9 and AV1 store the same quality in fewer bits than H.264, so matching that quality takes more space in an MP4.
Is the WebM uploaded anywhere?
No. The file is read in this tab; the only network request is the converter engine itself.
What happens to the audio?
The first audio track, usually Opus or Vorbis, is decoded and encoded as AAC. Additional tracks are not copied.
Can I convert several WebM files at once?
The page converts one file per run. For a batch, run them one after another or use a desktop tool that accepts a queue.