Free converter · Lossy re-encode
Convert MOV to WebM
Short answer
Drop your MOV 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
- This is a re-encode: video becomes VP8 and audio Opus, with ffmpeg's default settings for the video.
- Measured on a 1080p clip: 9.18 MB in, 0.83 MB out (659 kbps), SSIM 0.894 against the original. That is a visible quality drop.
- Matched file size matters: VP9 at CRF 32 (2.37 MB) scored SSIM 0.9846 on the same clip, VP8 at 2 Mbps (2.39 MB) scored 0.9732.
- TikTok's API and LinkedIn list WebM; YouTube recommends MP4; Pinterest's web uploader lists MP4 and M4V only.
Measured: converter defaults against better settings
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.
SSIM is measured against the MOV's video (1.0 is identical). The first row is exactly what this page's converter does.
| Settings | File size | Average bitrate (whole file) | SSIM | Encode time (native) |
|---|---|---|---|---|
| Converter default: VP8, Opus | 825,265 bytes | 660 kbps | 0.8942 | 8.3 s |
| VP8 at 2,000 kbps | 2,391,984 bytes | 1,913 kbps | 0.9732 | 14.2 s |
| VP9, CRF 32 | 2,368,239 bytes | 1,895 kbps | 0.9846 | 21.3 s |
| For reference: H.264 CRF 23, AAC | 3,349,557 bytes | 2,680 kbps | 0.9887 | 6.3 s |
At about the same size, VP9 beat VP8 by 0.0114 SSIM on this clip. The converter's VP8 default is small and fast to start, not high quality.
MOV vs WebM
| MOV | WebM | |
|---|---|---|
| Full name | QuickTime File Format (Apple) | WebM (open web format) |
| What is inside | H.264, HEVC or ProRes video with AAC or PCM audio | VP8, VP9 or AV1 video with Vorbis or Opus audio |
| Compression | Depends on the codec inside | Lossy |
| Best for | iPhone recordings and Final Cut Pro projects | 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. |
Getting more quality from the terminal
ffmpeg -i in.mov -c:v libvpx-vp9 -crf 32 -b:v 0 -row-mt 1 -c:a libopus -b:a 128k out.webm(the VP9 row above).ffmpeg -i in.mov -c:v libvpx -b:v 2M -auto-alt-ref 0 -c:a libopus out.webm(the VP8 at 2 Mbps row).- Lower CRF numbers mean higher quality and bigger files; WebM's own documentation lists VP8 and VP9 as the video codecs and Vorbis or Opus as audio.
Time in the browser
The converter runs a single-thread WebAssembly build. On our server the VP8 and Opus conversion of a 3 second 1080p clip took 30.6 seconds, against 8.3 seconds for the 10 second clip with native ffmpeg. Expect a long wait for large files; the converter accepts up to 100 MB.
Questions people ask
Is this MOV 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 MOV 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.
Does MOV to WebM lose quality?
Yes, it is a re-encode. With this converter's defaults, our 1080p test clip fell to SSIM 0.894 at 659 kbps.
Why is my WebM so small?
The converter uses ffmpeg's default VP8 settings, which target a low bitrate.
Is WebM accepted for uploads?
TikTok's Content Posting API and LinkedIn list WebM. YouTube recommends MP4, and Pinterest's web uploader lists MP4 and M4V only.
What audio does WebM use?
The WebM Project lists Vorbis or Opus; this converter writes Opus.
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)
- TikTok for Developers: Content Posting API media transfer guide (read 2026-10-08)
- LinkedIn Help: Video sharing troubleshooting (supported file types) (read 2026-10-08)
- Pinterest Help: Review Pin specs (Video Pin file types) (read 2026-10-08)
- YouTube Help: Recommended upload encoding settings (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.