Reviewed 2026-10-20 by the Spool editorial team · How we test
MKV and MP4: same contents, different box
Matroska (MKV) is a container, not a codec. Most MKV files you'll meet hold H.264 or HEVC video, which MP4 can also hold. In that case we copy the video stream into an MP4 without re-encoding it, which takes seconds and changes nothing in the picture. Only the audio may need work: MP4 doesn't officially support Opus, FLAC, Vorbis, DTS or TrueHD, so those are converted to AAC while the video is copied untouched.
| Video in the MKV | Audio in the MKV | Result |
|---|---|---|
| H.264, HEVC or AV1 | AAC, AC-3, E-AC-3 | Both copied — lossless, seconds |
| H.264, HEVC or AV1 | Opus, FLAC, Vorbis, DTS, TrueHD, MP3 | Video copied, audio encoded to AAC (192 kbps by default) |
| VP9, VP8, MPEG-4 Part 2, MPEG-2 and others | Any | Video encoded to H.264, audio to AAC |
Important: what an MP4 can't take with it
MKV's big strength is holding many tracks at once: several audio languages, commentary, and subtitle tracks. Our MP4 output keeps one video track and the first audio track, and leaves subtitle tracks out. That's what phones, editors and social networks expect, but it means:
- If the MKV is a film with several languages, the MP4 has the first one (usually the original language).
- Soft subtitles (SRT, ASS, PGS) are not carried over or burned in. Keep the MKV if you rely on them.
- OBS recordings with separate tracks for game audio, microphone and Discord keep only track 1, the mixed track by default.
OBS users: why your recordings are MKV
OBS Studio records to MKV by default because Matroska survives crashes: if OBS, the game or Windows freezes mid-recording, an MKV is still playable up to that point, while an unfinished MP4 is usually unreadable. Recording to MKV and converting afterwards is the right workflow. OBS also has a built-in File → Remux Recordings option that does the same stream copy on your PC. Use this page when you're on another computer, on your phone, or when the audio needs converting to AAC for an editor.
OBS records at a constant frame rate, so a remuxed OBS file drops straight into Premiere Pro, DaVinci Resolve, CapCut or iMovie.
When to keep MKV instead
- You watch in VLC, Plex, Jellyfin or Kodi; they all play MKV natively.
- You need multiple audio or subtitle tracks.
- You're archiving and want everything in one file.
Convert to MP4 when the file has to work on an iPhone, a smart TV's USB player, in Premiere or Final Cut, or on social networks, which mostly reject MKV uploads.
Surround sound
AC-3 and E-AC-3 (Dolby Digital) 5.1 audio is copied as-is, which smart TVs and home cinema players handle well. Some browsers and phones can't play AC-3 though. If your MP4 plays without sound in a browser, convert again and choose an Audio bitrate: that forces AAC, which plays everywhere.
Troubleshooting
| Problem | Explanation and fix |
|---|---|
| Conversion takes minutes, not seconds | The video is VP9 or another codec MP4 players don't expect, so it's being encoded. That's normal; progress is shown live. |
| Wrong language in the MP4 | We keep the first audio track. Remux the MKV with a tool such as MKVToolNix to reorder tracks first. |
| Subtitles missing | Subtitle tracks are not included in the MP4 output. |
| No sound in browser | AC-3 audio was copied. Re-convert with an audio bitrate selected to get AAC. |