Free · no signup · never uploaded
WEBM to MP4 Converter
Turn a WebM into an MP4 that plays on anything — in your browser.
Drop a WEBM here or choose a file
or drop your file here.
WEBM · converted in your browser, never uploaded
Runs in your browser — your file is never uploaded.
Short answer
To convert WEBM to MP4, drop your file below and download the MP4. WebM holds VP8/VP9 video, which MP4 can’t carry, so this one is a real re-encode to H.264 — slower than a rewrap, and still entirely in your browser.
Why this conversion can’t be instant
Rewrapping only works when the video inside is already a codec the new container supports. WebM holds VP8 or VP9; MP4 in practice needs H.264. There’s no way to copy one into the other, so every frame must be decoded from VP9 and re-encoded as H.264. That’s genuine computation — which is why this tool accepts smaller files than the rewrapping ones, and why it takes minutes rather than seconds.
Where WebM files come from
WebM is a web-native format: it’s what many browser screen recorders, WebRTC captures, and canvas or MediaRecorder exports produce, and it’s common in downloaded web video. It plays fine in browsers — but drop one into a video editor, a phone’s gallery, or a social upload form and it’s often rejected. Converting to MP4 is usually the shortest path to making that recording usable.
Video formats to MP4 — what happens to your file
| Input format | Usual codecs inside | Conversion | Max file size |
|---|---|---|---|
| MOV (QuickTime) | H.264 + AAC | Rewrap (lossless) | 500 MB |
| MKV (Matroska) | H.264 + AAC | Rewrap (lossless) | 500 MB |
| WebM | VP8/VP9 + Opus | Re-encode to H.264 | 100 MB |
| AVI | MPEG-4/DivX | Re-encode to H.264 | 100 MB |
| GIF | Animated GIF | Re-encode to H.264 | 200 MB |
A rewrap copies the existing video and audio streams into an MP4 container untouched — it is lossless and finishes in seconds. A re-encode decodes and re-compresses, so it takes longer and loses a little quality. Size caps differ because everything runs on one thread in your browser.
How this conversion works
Everything happens locally in your browser with ffmpeg — here is exactly how:
- Load the engine. ffmpeg compiled to WebAssembly loads once from this site — your file is never uploaded.
- Decode VP8/VP9. the WebM video is decoded with libvpx, since MP4 cannot carry VP8 or VP9 streams as-is.
- Re-encode to H.264. frames are encoded with libx264 (veryfast preset, single thread) and the audio is re-encoded to AAC for MP4 compatibility.
- Write the MP4. the result is muxed into an MP4 with the moov atom at the front so it plays immediately.
Assumptions
- Everything runs client-side; this is a full re-encode, so it is much slower than a rewrap.
- Re-encoding between lossy codecs costs a small amount of quality.
- Speed depends on your device, the clip’s length, and its resolution.
Last reviewed: July 19, 2026
Frequently asked questions
How do I convert a WEBM to MP4?+
Drop your .webm in above and press convert. The video is re-encoded from VP8/VP9 to H.264 in your browser — nothing is uploaded. Expect it to take a while; this is a real re-encode.
Are my videos uploaded to a server?+
No. Everything runs 100% in your browser using ffmpeg compiled to WebAssembly, so your file never leaves your device. Nothing is uploaded, stored, or sent anywhere — and once the converter has loaded it even works offline.
Why is this slower than MOV to MP4?+
MOV usually already contains H.264, so it can just be rewrapped. WebM contains VP8 or VP9, which MP4 can’t hold, so every frame has to be decoded and re-encoded.
Does it lose quality?+
A little. Re-encoding from one lossy codec to another always costs some detail, but the tool encodes at a high-quality setting so it’s usually not visible at normal viewing size.
Why is the first conversion slower?+
The first time, your browser downloads the ffmpeg engine (about 31 MB) and starts it up. That happens once — later conversions reuse the loaded engine and are much faster.
Why is the file-size limit lower here?+
Because every frame must be re-encoded rather than copied. Because it runs on a single thread inside your browser, speed depends on your device, the clip’s length, and its resolution. The limit keeps the page from freezing on a file it can’t realistically finish.
Related tools
Video is processed entirely in your browser with ffmpeg and never uploaded.