Convert MP4 to MKV online
Repackage an MP4 video into an MKV container without re-encoding it, ready for Plex, a media server, or adding external subtitles, without uploading anything to a server.
Convert MP4 to MKV
Your file is processed right in this browser: it never gets uploaded to any server.
Why use PullVid for MP4 → MKV?
Conversion details
How do you convert an MP4 to MKV?
Choose the MP4 file you want to convert to MKV.
Hit 'Convert'; since it's a remux (no video re-encoding), it usually takes only a few seconds.
Download the resulting MKV; it saves straight to your device.
If you have an MP4 video and need to add external subtitles, keep multiple audio tracks, or upload it to a media server like Plex, this converter repackages it into MKV without touching the video. It all happens in your own browser: we never upload the file to a server.
What's converting an MP4 to MKV good for?
MP4 is the most compatible container out there, but it falls short on a few things MKV handles natively: multiple audio tracks in a single file, subtitles in advanced formats (ASS/SSA, with custom styling and positioning) and better management in media servers like Plex or Jellyfin, which use MKV as their reference format for large libraries. If you need any of that, converting the MP4 to MKV is the first step.
How it works: instant remux, not re-encoding
We use ffmpeg compiled to WebAssembly with direct stream copy ('-c copy'): the MP4's video and audio get repackaged as-is inside an MKV container, without decompressing or recompressing anything. That's why the process takes seconds even with files several GB in size, and the result matches the original in quality exactly.
Steps to convert your MP4 to MKV
- Choose the MP4 file from your device ('Choose file' button).
- Hit 'Convert'. The first time, your browser downloads the conversion engine (a few MB); after that, it's faster.
- Wait a few seconds for the remux to finish.
- Download the resulting MKV with the 'Download result' button.
Why MKV is better for subtitles and multiple audio tracks
Unlike MP4, the MKV container supports subtitles embedded in ASS/SSA format (with custom colors, positioning and styling, typical of fansubs) and several simultaneous audio tracks — original language plus a dub, for instance — with no need to re-encode anything. It's also the format managers like Plex or Jellyfin prefer for organizing video libraries with rich metadata.
Honest limits worth knowing
Since the process runs in your browser, the real limit is your device's memory when loading the file, not an artificial cap of ours. With very large files (several GB), loading can take longer, especially on phones with limited RAM — try a computer if that happens. The remux itself is nearly instant.
Other related conversions
If you need the reverse conversion, use MKV to MP4 to get back universal compatibility. And if you're not sure which container to stick with, check our guide MP4 vs MKV: which one should you choose. If you don't have the video yet, use our YouTube video downloader.
Related articles
MP4 vs MKV: which format to choose when downloading a video
Choose MP4 for maximum compatibility (mobile, social, any player) and MKV only for 4K or videos with multiple audio/subtitle tracks. Here's the difference.
MP4, WebM, MKV: Which Video Format to Choose When Downloading
Video format comparison: MP4, WebM, MKV, and AVI. Find out which one offers the best quality and compatibility for your downloads.
Convert a video to MP4: how to download it already in this format
Need a video in MP4? With PullVid you download it straight in MP4, the most compatible format. No conversion software: paste the link, pick MP4, and you're done.
Frequently asked questions about MP4 → MKV
What's the point of converting an MP4 to MKV if it already plays fine?
Mostly for what the MKV container supports that MP4 doesn't handle as well: multiple audio tracks at once (original audio plus a dub, say), externally embedded subtitles without re-encoding, and better support in media servers like Plex or Jellyfin, which favor MKV for organizing large libraries.
Do I lose quality converting an MP4 to MKV?
No. It's a remux: we copy the exact same video and audio streams into a different container (ffmpeg's '-c copy' option), without decompressing or recompressing anything. The result is identical in quality, bit for bit.
Why pick MKV instead of just keeping the MP4?
If you're planning to add external subtitles, keep several audio tracks in one file, or feed it to a Plex/Jellyfin server, MKV handles that better. If you just need universal compatibility for playback or sharing, the original MP4 already does the job.
Is there a size limit on the MP4 file?
We don't set an artificial limit, but the process runs in your browser: files above 2 GB can use a lot of memory just to load, especially on phones with limited RAM. The remux itself is nearly instant; loading the file is usually the bottleneck.
I don't have the MP4 file yet — can I download it here?
This converter works with a file you already have saved. If you first need to download the video, use our YouTube video downloader, which delivers directly in MP4; then come back here to convert it to MKV.