Free converter · Lossy re-encode
Convert MP4 to WebM
Short answer
Drop your MP4 file into the converter below and download the WebM file. It runs in your browser with FFmpeg, for files up to 100 MB; nothing is uploaded. The file is re-encoded to VP8 video and Opus audio in WebM. Re-encoding takes longer than a container swap and costs some quality; how much depends on the clip and on the quality setting, and at the converter's default setting the loss can be visible, so keep the original.
Maximum file size: 100 MB. The first conversion loads a ~30 MB converter engine, once per visit. Nothing is uploaded - conversion runs on your device.
H.264/AAC re-encoded to VP8/Opus. · Lossy re-encode · Browser-side via FFmpeg WASM - nothing uploaded.
Key takeaways
- MP4 to WebM re-encodes: H.264 becomes VP8 and AAC becomes Opus.
- Measured on one 1080p clip: 9.18 MB MP4 became 825 KB (659 kbps) with SSIM 0.894.
- For web pages, WebM is a delivery choice; for uploads, check the platform: YouTube recommends MP4, TikTok's API and LinkedIn list WebM.
- Choose by size and quality target, not by format name: at about 2.4 MB, VP9 scored 0.9846 and VP8 0.9732.
The numbers for this conversion
Measured on a 10 second, 1920 × 1080, 30 fps excerpt of Big Buck Bunny (animated film, not camera footage; (c) Blender Foundation, CC BY 3.0) as H.264 plus stereo AAC 192 kbps, using the same ffmpeg arguments this converter passes to its WebAssembly build, run with ffmpeg 6.1.1. Results describe this file, not every file.
| Output | Size | SSIM vs MP4 | Time (native / 3 s clip in the browser core) |
|---|---|---|---|
| Converter: VP8 + Opus defaults | 825,285 bytes (659 kbps) | 0.8942 | 8.3 s / 30.6 s |
| VP8 at 2 Mbps | 2,391,984 bytes | 0.9732 | 14.2 s / not run |
| VP9 CRF 32 | 2,368,239 bytes | 0.9846 | 21.3 s / not run |
The browser column is the shipped single-thread WebAssembly core on a 3 second excerpt, so it is slower than native ffmpeg; the two are different clip lengths.
MP4 vs WebM
| MP4 | WebM | |
|---|---|---|
| Full name | MPEG-4 Part 14 | WebM (open web format) |
| What is inside | H.264 or H.265 video with AAC audio | VP8, VP9 or AV1 video with Vorbis or Opus audio |
| Compression | Depends on the codec inside | Lossy |
| Best for | Uploading anywhere: the format social platforms document first | Video on web pages; YouTube accepts it, most short-form apps do not |
What you get
VP8 video and Opus audio in WebM. H.264/AAC re-encoded to VP8/Opus.
Where you can upload WebM
| Platform | WebM | What the platform documents |
|---|---|---|
| TikTok | Partly | Yes through the TikTok Content Posting API - native upload support depends on the surface. |
| Instagram Reels | Not documented | WebM is not in Instagram's published video specs, which list MOV and MP4. Convert to MP4 before uploading. |
| Instagram Feed | Not documented | WebM is not in Instagram's published video specs, which list MOV and MP4. Convert to MP4 before uploading. |
| YouTube Shorts | Accepted | Yes - WebM is on YouTube's own list of supported upload formats. |
| X (Twitter) | Not accepted | WebM is not listed for X Media Studio uploads. |
| Facebook Reels | Unclear | Current WEBM support for Facebook video uploads has not been reverified. |
| Not accepted | No - WebM isn't in Pinterest's accepted list on either the app or the web. |
When WebM makes sense
WebM is an open container for VP8, VP9 or AV1 video with Vorbis or Opus audio, according to the WebM Project. It suits video embedded in web pages. For social uploads, documentation favors MP4: YouTube recommends MP4 with H.264, TikTok's API lists MP4 as recommended (WebM and MOV also accepted), and Pinterest's web uploader lists MP4 and M4V only.
Better terminal settings
ffmpeg -i in.mp4 -c:v libvpx-vp9 -crf 32 -b:v 0 -row-mt 1 -c:a libopus -b:a 128k out.webm- Raise quality by lowering the CRF number; the FFmpeg wiki's scale for x264 does not apply to VP9, so test two or three values on your own file.
Questions people ask
Is this MP4 to WebM converter free and private?
Yes. It is free, needs no account and runs entirely in your browser, so the file never leaves your device.
Will converting MP4 to WebM reduce quality?
Some, because WebM is lossy and the file is re-encoded. How much depends on the clip and on the quality setting, and at the converter's default setting the loss can be visible, so keep the original. For uploads, the platform re-encodes again anyway.
Is there a file size limit?
Files up to 100 MB. The first conversion downloads a converter engine of about 30 MB once per visit; larger files are better handled by a desktop app such as HandBrake or FFmpeg.
Is MP4 to WebM lossless?
No. It is a re-encode, and the converter's default VP8 settings are low bitrate.
How big will the WebM be?
For our 1080p, 10 second clip: 825 KB with this converter, or about 2.4 MB with VP9 CRF 32 or VP8 at 2 Mbps.
Does WebM have audio?
Yes. This converter writes Opus audio; the WebM Project lists Vorbis or Opus.
Which is better, VP8 or VP9?
At about 2.4 MB, VP9 scored SSIM 0.9846 and VP8 0.9732 on our clip. It took longer to encode: 21.3 s against 14.2 s natively.
More converters
Sources and how we checked
- MDN: Media container formats
- MDN: Web audio codec guide
- MDN: Image file type and format guide
- FFmpeg documentation: stream copy
- The WebM Project: WebM container guidelines (codecs) (read 2026-10-08)
- FFmpeg Documentation: ffmpeg (stream selection, -map, -c copy) (read 2026-10-08)
- YouTube Help: Recommended upload encoding settings (read 2026-10-08)
- TikTok for Developers: Content Posting API media transfer guide (read 2026-10-08)
- Pinterest Help: Review Pin specs (Video Pin file types) (read 2026-10-08)
- Blender Foundation: Big Buck Bunny (CC BY 3.0), the clip used for the measurements (read 2026-10-08)
Reviewed October 8, 2026. What the converter writes was measured with ffprobe on the same FFmpeg build your browser downloads; format descriptions follow MDN's media guides.