Free converter · Lossy re-encode
Convert WebM to MP4
Short answer
Drop your WebM file into the converter below and download the MP4 file. It runs in your browser with FFmpeg, for files up to 100 MB; nothing is uploaded. The file is re-encoded to H.264 video and AAC audio in MP4, at FFmpeg's default quality, with the source's sample rate. 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.
VP8/Opus re-encoded to H.264/AAC. · Lossy re-encode · Browser-side via FFmpeg WASM - nothing uploaded.
Key takeaways
- WebM to MP4 re-encodes: VP8 or VP9 becomes H.264, Opus or Vorbis becomes AAC.
- Measured: a 2.37 MB VP9 and Opus WebM became a 3.05 MB MP4 (+28.8%) at SSIM 0.9802 against the original video.
- The converter uses ffmpeg's default H.264 settings (CRF 23) and writes yuv420p, which players accept.
- YouTube recommends MP4 with H.264; Pinterest's web uploader lists MP4 and M4V only.
Measured: size and quality
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.
The WebM input was made from the same excerpt with VP9 CRF 32 and Opus 128 kbps.
| Check | Result |
|---|---|
| Input | VP9 + Opus WebM, 2,368,239 bytes |
| Output | H.264 yuv420p + AAC MP4, 3,050,245 bytes (+28.8%) |
| Output video bitrate | about 2,302 kbps (AAC 128 kbps) |
| SSIM against the original MP4 video | 0.9802 |
| SSIM against the WebM it was made from | 0.9734 |
| Time, native ffmpeg | 4.8 s |
| Time, converter's WebAssembly core, 3 s clip | 32.6 s |
The MP4 is bigger because H.264 needs more bits than VP9 for similar quality, and a re-encode never recovers detail the WebM already dropped.
WebM vs MP4
| WebM | MP4 | |
|---|---|---|
| Full name | WebM (open web format) | MPEG-4 Part 14 |
| What is inside | VP8, VP9 or AV1 video with Vorbis or Opus audio | H.264 or H.265 video with AAC audio |
| Compression | Lossy | Depends on the codec inside |
| Best for | Video on web pages; YouTube accepts it, most short-form apps do not | Uploading anywhere: the format social platforms document first |
What you get
H.264 video and AAC audio in MP4, at FFmpeg's default quality, with the source's sample rate. VP8/Opus re-encoded to H.264/AAC.
Why convert to MP4
Documentation favors MP4: YouTube recommends MP4 with H.264 video and AAC-LC or Opus audio; Pinterest's web uploader lists MP4 and M4V only. TikTok's Content Posting API lists WebM, but marks MP4 as the recommended container.
Terminal version with quality control
ffmpeg -i in.webm -c:v libx264 -crf 20 -pix_fmt yuv420p -c:a aac -b:a 160k -movflags +faststart out.mp4- The converter's default is CRF 23. The FFmpeg wiki says the scale runs 0 to 51, with 17 to 28 a sane range, and +6 roughly halves the size.
Questions people ask
Is this WebM to MP4 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 WebM to MP4 reduce quality?
Some, because MP4 is depends on the codec inside 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 WebM to MP4 reduce quality?
It re-encodes, so some. Our test scored SSIM 0.9802 against the original video, and the file grew 28.8%.
Why is the MP4 bigger than the WebM?
H.264 generally needs more bits than VP9 for the same quality, and the converter's default settings do not target the WebM's size.
Will the MP4 play on iPhones and in editors?
It is H.264 in yuv420p with AAC, the common combination; check your target's documentation.
Can I keep the WebM's quality exactly?
No format change between different codecs is lossless. Keep the WebM if it is accepted.
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
- FFmpeg Wiki: Encode/H.264 (CRF and presets) (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)
- The WebM Project: WebM container guidelines (codecs) (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.