WebM to MP4
Turn a WebM file into an MP4 that plays everywhere. The conversion runs on your own device — your file is never uploaded, and you choose the output size in the same pass.
Drop a video here, or browse
MP4 · MOV · WebM — compressed right here, nothing uploaded
100% on-device · no signup · no watermark
Loading compressor…
Bigger files take longer — your CPU does the work, privately.
WebM has to be re-encoded, not renamed
A WebM file carries VP8, VP9 or AV1 video and Opus or Vorbis audio. An MP4 that plays everywhere carries H.264 video and AAC audio. The two share no codec, so nothing can be copied across: renaming the file to .mp4 produces a file most players refuse, and every converter — this one included — has to decode the video and encode it again from scratch.
That is worth knowing before you pick a tool, because it means the work is identical wherever it happens. A server-side converter is not doing something your machine cannot; it is doing the same re-encode on someone else’s CPU, after your file has been uploaded to it.
Converted on your device, not on a server
Drop a .webm file above and the conversion runs inside this page with WebAssembly. Nothing is uploaded, so there is no queue, no daily conversion count, and no service-side file-size ceiling — the limit is your own browser’s memory. The engine downloads once from a public CDN and is then cached, so a second conversion works even with the network off.
The output is a standard H.264 + AAC MP4 with the moov atom at the front, which is what makes it start playing immediately when it is streamed or previewed. If you only wanted a smaller file and the format was never the problem, compress the WebM instead.
Choose the output size in the same encode
Because a re-encode is unavoidable, you may as well decide how big the result should be while it happens. Drag the target-size slider (2 MB to 200 MB) before you start: set it high and the converter keeps close to the source detail, set it low and you get a conversion and a compression in one pass instead of converting first and shrinking the MP4 afterwards, which would re-encode your video a second time and cost you quality twice.
One caveat worth stating plainly: at very low targets the encoder lowers the resolution to keep the picture from breaking up — below roughly 1 Mbit/s it drops to 480p, below 2.5 Mbit/s to 720p. If you want the source resolution kept, leave the target high. For the reasoning behind that trade-off, see how to keep as much quality as possible.
Frequently asked questions
How do I convert WebM to MP4 for free?
Drop the .webm file above, set the target size, and press compress — the file is decoded and re-encoded to an H.264 MP4 inside this page, then downloaded. There is no signup, no watermark and no conversion count.
Can I just rename .webm to .mp4?
No. The extension does not change what is inside the file. A WebM holds VP8, VP9 or AV1 video, which an MP4 player does not expect, so a renamed file will fail to open or play with no picture. The video has to be re-encoded to H.264.
Is my file uploaded to convert it?
No. The conversion runs in your browser with WebAssembly, so the file never leaves your device. Only the engine binary is fetched, once, from a public CDN — after that the page works offline.
Does converting WebM to MP4 lose quality?
A little, because VP9 and H.264 are different codecs and the video must be decoded and encoded again. Keep the target size high and the loss is hard to see; a low target trades more detail for a smaller file, and very low targets also reduce the resolution.
Why convert to MP4 at all?
MP4 with H.264 is the format almost everything accepts: iPhone and Android photo libraries, iMovie and most editors, PowerPoint and Keynote, and messengers and upload forms that reject WebM outright. WebM plays well in browsers and poorly almost everywhere else.
Is there a file size limit?
There is no limit imposed by a service, because there is no service — but your browser has to hold the file in memory. Very large files can exhaust that; if a conversion fails, trim the clip with the built-in trim control first or close other tabs.