OneTaskBy Tim King · Updated
WebM to MP4
Screen Recordings and Web Clips Converted for iPhones, Editors and TVs
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How the WebM To MP4 works
This WebM to MP4 converter turns a WebM file into an MP4 with H.264 video and AAC sound. That is the format iPhones, video editors, TVs and upload forms expect. Browser screen recorders, Discord clips and many web downloads produce WebM, and this page converts them on your device. Nothing is uploaded.
WebM to MP4: why the video has to be re-encoded
Google launched WebM in 2010 as a royalty-free format for the web. It is a cut-down version of the Matroska (MKV) container and only allows open codecs: VP8, VP9 or AV1 for video, and Vorbis or Opus for sound. MP4 players expect H.264 video and AAC sound. So, unlike a MOV to MP4 conversion, the streams cannot simply be moved across. The page decodes each frame and encodes it again as H.264, and converts the sound to AAC.
Where WebM files come from
- Browser recordings. Chrome and Firefox record screen, camera and tab captures as WebM, so most web-based screen recorders and meeting tools save in it.
- Web downloads. YouTube and other sites serve VP9 or AV1 in WebM, and download tools often keep it.
- Chat and game clips. Discord, OBS presets and some clip tools save WebM.
How it runs
The page reads the WebM with Mediabunny, an open-source media library. It then uses WebCodecs, the browser's interface to your device's video hardware, to decode VP9 and encode H.264. That usually runs several times faster than real time. Browser recordings sometimes lack a duration in the file header. The page works it out from the frames, so the progress bar still moves.
Quality and size
H.264 needs a somewhat higher bitrate than VP9 for the same picture, so the MP4 often comes out a little larger than the WebM. The encoder runs at a high quality setting so the result looks the same. To shrink it for sending, run the MP4 through the MP4 compressor. To keep only the sound, use WebM to MP3.
FAQ
Why is the MP4 bigger than the WebM?
VP9 compresses better than H.264, so matching its quality takes more bits. The difference is usually 10 to 50 percent. The MP4 compressor can bring it back down with a target size.
My WebM has no sound in the MP4. Why?
Either the WebM had no sound track, or it used a codec this browser cannot decode. The results panel says which. Opus and Vorbis sound decode in Chrome, Edge and current Safari.
Does it work with transparent WebM files?
The video converts, but MP4 with H.264 has no transparency, so transparent areas become black. Keep the WebM for web pages that need the see-through background, since VP9 WebM supports an alpha channel.
Is the file uploaded?
No. The conversion runs in this browser tab and the video never leaves your device. The page only needs the internet to load itself and the converter library, and the finished file is saved straight from memory to your downloads.
See also: all Audio & Video tools, Units, Images, Documents, Data & Code.
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