Video converter — to MP4 (H.264)

How to use it

1. Choose a video — MOV, MKV, WebM, AVI, MP4 and more all work.
2. The output is MP4 (H.264) — the format that plays on virtually every device and platform.
3. Press Convert — the first run downloads the engine once, then the converted video downloads.

A container is not a codec, and converting often changes only one

The extensions .mp4, .mkv, .mov and .avi name containers, filing systems that hold streams and record where each begins. H.264, HEVC, AAC and MP3 name codecs, the schemes the streams inside are compressed with. A .mov off a camera and a .mp4 off a phone very often hold the same H.264 video: the wrapper differs, the picture data does not. This is why a file that will not play sometimes needs only a different container.

Repackaging streams without touching them is called remuxing. It is fast and exactly lossless, the same class of operation as stripping an audio track. Re-encoding decodes every frame and compresses it again from nothing, which is where the time goes and a generation of quality with it. Two files both described as converted to MP4 can be products of these two very different processes.

Why H.264 is the destination rather than one option among several

The encoder set compiled into this browser build is H.264 for video, with AAC, MP3, Opus, Vorbis and FLAC for audio. H.264 has hardware decoding in effectively every phone, laptop, television and browser shipped over the past fifteen years, and it is what editors and upload pipelines expect on ingest. Several newer codecs compress better; none plays as dependably, and dependable playback is usually the entire reason a file is being converted.

The cost is that a full re-encode is genuine computation, done here by ffmpeg compiled to WebAssembly on a single thread, with no multi-core split available. Expect several times longer than the same job on a desktop command line, and note the file occupies the tab’s memory throughout. Those two facts together put the practical ceiling well below a desktop encoder’s, making this a tool for clips rather than feature-length footage.

Questions