WebM to MP4 Converter
Convert WebM to MP4 on your device. Create H.264 MP4 with AAC audio. Preview and download the result without uploading your video.
Up to 100 MB. The conversion engine downloads on first use (~31 MB).
How to convert WebM to MP4
Choose or drop a WebM file, select a video quality setting and click Convert. The page downloads its conversion engine on first use, shows progress and then processes the file on your device. Preview the result and download it.
The input is limited to 100 MB. Conversion may take longer than the media duration, particularly on phones. Cancel stops the engine and frees its working memory; you can select a smaller file or retry afterward.
What the conversion changes
WebM video is re-encoded to H.264 video in an MP4 container. When present, the first audio track is encoded as AAC. The result uses YUV 4:2:0 pixel format and fast-start layout for broad playback compatibility.
CRF controls quality: a lower value generally gives higher quality and a larger file. Odd pixel dimensions are padded by at most one pixel for encoder compatibility. Alpha transparency is not preserved in the H.264 result; subtitles and extra tracks are omitted.
Engine download, privacy and failures
The page uses the single-threaded FFmpeg WebAssembly engine. Its version-pinned JavaScript and WebAssembly files are downloaded from jsDelivr when conversion starts. Your selected media stays on this device and is not uploaded to the CDN.
Processing needs memory for the compressed input, decoded media and output. Browser memory pressure, unsupported codecs or a missing required track can cause failure even for a file below 100 MB. A conversion is stopped after five minutes; use a smaller clip or a desktop encoder for larger jobs.
Questions & answers
Will renaming the file extension work?
No. Renaming does not change the encoded streams. This tool performs an actual conversion and checks the encoder exit status before offering a download.
Why does the first conversion load slowly?
The conversion engine is about 31 MB and is loaded only when needed. The status displays download progress; browser caching can help subsequent visits.
Can I cancel or convert offline?
You can cancel an active conversion. The engine must be downloaded before processing; this page does not promise offline availability across new visits.