MP4 to MKV
MP4 is great for compatibility, but it is limited when you want to bundle multiple audio languages, several subtitle tracks, and chapter markers into one clean file. MKV, the open Matroska container, is built for exactly that. Converting MP4 to MKV is usually a fast, lossless remux: your H.264 or H.265 video and AAC audio are copied straight into the more flexible Matroska container with no quality change at all. Once inside MKV, you can add extra tracks and softcoded subtitles that MP4 struggles to hold. FluidConvert rewraps your MP4 into MKV in your browser, free, with no account and no watermark, giving you a container designed for archiving and richer multi-track video.
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Accepts: video/mp4, .mp4 · Max 1GB · always free
Your files are encrypted with TLS and automatically deleted after conversion.
Simply upload your .MP4 file and we'll convert it to .MKV format — fast, free, and secure.
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Convert files up to 1GB completely free. No account, no limits.
Secure
Files are encrypted and automatically deleted after conversion.
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Industry-leading conversion with no quality loss.
How to Convert MP4 to MKV
Upload Your File
Click the upload area above or drag and drop your .MP4 file. We support files up to 1GB, completely free.
Choose Output Format
Select .MKV as your target format. Adjust any conversion settings if needed.
Download Your File
Click Convert Now and wait a few seconds. Once complete, download your converted file instantly.
About MP4 to MKV
Last updated: August 2026
What is MP4?
MP4 (MPEG-4 Part 14) is the most widely supported video container in the world, standardized in 2001. It typically wraps H.264 or H.265 video with AAC audio and plays natively on nearly every phone, browser, TV, and app. Its strengths are compatibility, efficient streaming, and small file size, which is why cameras, editors, and streaming services default to it. Its limitations show up with complex multimedia: MP4 handles multiple audio tracks awkwardly and doesn't support the flexible softcoded subtitle and chapter systems that media enthusiasts and archivists often want in a single file.
Why convert to MKV?
MKV (Matroska) is the container of choice for people who want everything in one file. It can hold an unlimited number of video, audio, and subtitle tracks, chapter markers, cover art, and even lossless audio, all in an open, royalty-free format. Home media servers like Plex and enthusiasts building movie libraries favor MKV because a single file can carry multiple language dubs and subtitle options that viewers switch between on the fly. Converting MP4 to MKV gives you that flexibility and makes a cleaner container for archiving, without giving up the video quality of your source.
What to expect from the conversion
MP4 to MKV is typically a lossless remux. MKV accepts the H.264 or H.265 video and AAC audio already in your MP4, so FluidConvert copies those streams directly into the Matroska container without re-encoding. Quality is identical, file size is essentially unchanged, and processing is fast because no compression happens. Nothing is lost; you simply gain a more capable container. The video and audio play exactly as before, and the new MKV is ready for you to add extra subtitle or audio tracks in a media tool later, which the original MP4 couldn't accommodate as gracefully.
How FluidConvert handles it
Your MP4 streams are remuxed directly into an MKV container on secure servers, preserving the original video and audio with no re-encoding and no quality loss. Uploads are encrypted in transit and automatically deleted from our servers about an hour after processing. The conversion is free, needs no account, and adds no watermark.
Common reasons to convert MP4 to MKV
- Combining multiple audio languages and subtitle tracks into a single file for a home media library
- Preparing videos for a Plex or Kodi server, where MKV's multi-track support shines
- Archiving movies in an open, flexible container that can hold chapters and lossless audio
- Rewrapping an MP4 losslessly to add softcoded subtitles you can toggle on and off later
- Building a video collection where viewers can switch between dubbed audio tracks on the fly
Frequently Asked Questions
Does converting MP4 to MKV reduce quality?
No. This is a container change, not a re-encode. MKV supports the same H.264 or H.265 video and AAC audio your MP4 already contains, so FluidConvert copies those streams into the Matroska wrapper untouched. The picture and sound are bit-for-bit identical to the source, and the file size stays about the same. It is a lossless remux.
Why would I want MKV instead of MP4?
MKV can hold multiple audio languages, several softcoded subtitle tracks, chapter markers, and lossless audio in a single file, which MP4 handles poorly or not at all. If you are building a home media library, using Plex, or archiving movies with multiple dubs and subtitle options, MKV lets viewers switch tracks on the fly. It is the better container for rich, multi-track, archival video.
Will the MKV file be the same size as my MP4?
Essentially, yes. Because the conversion copies the existing video and audio streams without re-compressing them, the MKV ends up almost identical in size to the source MP4. Any tiny difference comes from container overhead, not from the video itself. You get a more flexible file with no meaningful size penalty.
Can I add subtitles or extra audio tracks after converting to MKV?
Yes, that is a key reason to use MKV. Once your video is in a Matroska container, tools like MKVToolNix let you mux in additional subtitle files and audio tracks that viewers can toggle. MP4's support for that is far more limited, so converting to MKV first makes building a multi-language, subtitled file much easier.
Will my MKV play on all my devices?
Not as universally as MP4. Plex, VLC, Kodi, and many smart TVs and media players handle MKV well, but Apple devices and some mobile apps don't support it natively. If you need playback everywhere, keep an MP4 copy for sharing and use the MKV for your archive and media server. MKV trades some device compatibility for far richer multi-track features.