To convert MOV or MKV to MP4, drop the file into a browser-based converter like MOV to MP4 or MKV to MP4. It runs a WebAssembly build of FFmpeg on your own device to repackage the video into an MP4 container, so nothing is uploaded. MP4 plays almost everywhere — phones, TVs, editors, and the web.
Why MOV and MKV won't play in some apps
MOV is Apple's QuickTime container, and MKV (Matroska) is popular for high-quality downloads and multi-track video. Both are perfectly good formats, but they're not universally supported. Windows apps, older TVs, presentation software, web uploaders, and many editing tools expect MP4. When a file "won't play" or a site rejects it, the problem is usually the container, not the video inside — and that's exactly what converting to MP4 fixes.
What "convert to MP4" actually does
Here's the useful mental model. A video file is a container (MOV, MKV, MP4) wrapping one or more streams — the video, the audio, sometimes subtitles. In many cases the video inside a MOV or MKV is already H.264, the same codec MP4 uses. Converting then means repackaging: the streams are copied into an MP4 container without being re-encoded. That's fast and completely lossless — the picture is identical.
When the inside codec isn't MP4-friendly (for example HEVC or a codec MP4 doesn't carry cleanly), the video has to be re-encoded, which takes longer and involves a small, normal quality trade-off like any video compression. Either way the result is a standard MP4 that plays where you need it.
How to convert, step by step
- Open MOV to MP4 for QuickTime files, or MKV to MP4 for Matroska files.
- Drop your video in — it's read into your browser, not sent anywhere.
- Wait for the conversion engine to load and run (see the note below), then download the MP4.
- Got a WebM file from a screen recorder instead? WebM to MP4 does the same job.
The honest note: FFmpeg loads once (~30 MB)
Browser-based video conversion is powered by FFmpeg compiled to WebAssembly. That engine is roughly 30 MB, and it downloads the first time you use a video tool in a session. That's a real, one-time wait — on a slow connection it's noticeable. After it's loaded, it's cached, so further conversions in the same session start straight away. We'd rather be upfront about this than pretend video tools are instant: the trade-off is a short initial load in exchange for your video never leaving your device.
Why do it in the browser at all
Once FFmpeg is loaded, everything runs locally. Your footage — family videos, unreleased work, client recordings — is never uploaded to a server, there's no file-size ceiling imposed by someone else's free tier, and there's no sign-up. It even works offline after the engine has cached. For large files this is often faster than a cloud converter too, because you skip the upload-and-download round trip entirely.
A note on file size
Converting MOV or MKV to MP4 changes the container, not necessarily the size. If your goal is a smaller file — to email or upload under a limit — repackaging alone won't shrink it much. Run the result through Compress Video afterwards, which re-encodes at a lower bitrate to cut the size, again entirely on your own device.