Step-by-step instructions
- 1
Understand why you're converting
MP4 to MKV is typically done for: adding subtitle files (SRT, SSA, or PGS subtitles can be embedded in MKV but not easily in MP4), adding multiple audio tracks (different languages, commentary tracks), embedding chapter information, or using the MKV format for a media server (Plex, Jellyfin, Kodi) that treats MKV as the preferred format for local libraries.
Go to converter - 2
Open FormatDrop's video converter or use MKVToolNix
For simple container remux (no re-encoding): FormatDrop's video converter (formatdrop.com/video-converter) handles MP4 to MKV. For advanced use (adding subtitle tracks, multiple audio tracks): MKVToolNix (free, cross-platform GUI tool at mkvtoolnix.download) provides fine-grained control over which streams to include and how they're arranged in the MKV.
- 3
Drop your MP4 file
Upload your MP4 file. For a pure container remux (no encoding), the conversion is extremely fast — near real-time — because no video data is decoded or re-encoded. A 10 GB MP4 remuxes to MKV in seconds with FFmpeg or MKVToolNix.
- 4
Select MKV as the output format
Choose MKV output. When remuxing without re-encoding, the video and audio streams are copied exactly — zero quality loss. The output MKV contains the same H.264 or H.265 video and AAC or AC3 audio as the source MP4, just wrapped in the MKV container.
- 5
Add subtitle files if needed (MKVToolNix)
In MKVToolNix: after adding your MP4 as source, also add your subtitle files (.srt, .ass, .ssa) as additional source files. They appear as separate tracks you can include in the output MKV. Set the language tag for each subtitle track. The resulting MKV has the video, audio, and subtitle tracks all embedded in a single file.
Why convert MP4 to MKV?
MKV (Matroska) was designed specifically as a future-proof multimedia container that could hold any video, audio, and subtitle format in a single file — a goal the MP4/MOV formats of the era didn't achieve as cleanly. MKV supports an unlimited number of audio and subtitle tracks, making it ideal for multi-language releases. It handles the embedded subtitle formats used by Blu-ray (PGS bitmap subtitles) and anime fansub groups (Advanced SubStation Alpha/ASS), neither of which MP4 handles well. For home theatre setups using Plex, Jellyfin, or Kodi: MKV is often the preferred format because it avoids transcoding when the video and audio codecs are already supported by the player, and the software has excellent MKV metadata support for artwork, ratings, and descriptions.
Your files never leave your device
FormatDrop runs the conversion engine entirely inside your browser using WebAssembly. No file upload. No server. Nothing stored. You can verify this by opening DevTools → Network tab and watching: zero upload requests.
Frequently asked questions
Does MP4 to MKV conversion lose quality?
Can I play MKV files on Apple TV or Roku?
How do I add subtitle tracks to an MKV file?
No account. No upload. Works in any browser.