Browser edition: files stay on your device. Up to 100 MB total and 10 minutes. First use downloads a 32 MB engine. Some formats depend on codec support. Public-link downloading is not ready on this free deployment.
Convert MP4 to MKV
Convert MP4 to MKV online for free. Smart remux when possible, high-quality encoding when not.
How to convert MP4 to MKV
- Add your fileDrop it on the page, choose it from your device, or paste it with Ctrl+V.
- ConvertAdjust settings if you like, then press the button. You'll see live progress.
- Browser processing uses original files on your device. Public-link downloading is unavailable.
What happens to your file
MP4 files are common for sharing, uploading to social networks, playing on any device, but not every app or device opens them. MKV is the better choice for archiving movies, OBS recordings, multi-language video. Spool converts MP4 to MKV with FFmpeg — real decoding and encoding, not a renamed file extension.
Convert and edit files on your device. The browser edition accepts up to 100 MB total, media up to 10 minutes and video up to 4K. Codec availability and device memory can limit individual jobs.
If your particular file uses a codec MKV can't hold, or you pick "High", "Smaller file" or a lower resolution, we re-encode with H.264 and AAC instead.
Settings
- Quality — High, Balanced, Smaller file. Balanced keeps the original streams when possible (no quality loss, very fast).
- Resolution — Same as source, 4K (2160p), 1440p, 1080p, 720p, 480p, 360p, 240p
- Audio bitrate — Auto · keep original if possible, 320 kbps · best, 256 kbps, 192 kbps · standard, 128 kbps · small, 96 kbps, 64 kbps · voice
MP4 vs MKV
| MP4 | MKV | |
|---|---|---|
| Full name | MPEG-4 Part 14 | Matroska Video |
| Created by | ISO / MPEG, 2001 | Matroska.org (open standard), 2002 |
| Type | Video container | Video container |
| Typical contents | H.264 or H.265 video with AAC audio | almost any video codec (H.264, HEVC, VP9, AV1) with any audio codec |
| Best for | sharing, uploading to social networks, playing on any device | archiving movies, OBS recordings, multi-language video |
| Strength | Plays everywhere, including iPhone, Android, Windows and macOS | Holds multiple audio and subtitle tracks |
| Weakness | Lossy: every re-encode loses a little quality | Not supported by iPhone, many smart TVs, Premiere and most social networks |
About MP4
- Type
- MPEG-4 Part 14
- Contains
- H.264 or H.265 video with AAC audio
- Use
- sharing, uploading to social networks, playing on any device
About MKV
- Type
- Matroska Video
- Contains
- almost any video codec (H.264, HEVC, VP9, AV1) with any audio codec
- Use
- archiving movies, OBS recordings, multi-language video
FAQ
Will converting MP4 to MKV reduce quality?
Not with the default settings in most cases: when the MP4 contains codecs MKV supports, Spool copies the streams without re-encoding, so quality is identical. A re-encode happens only if needed or if you change quality or resolution.
How long does it take?
A remux takes a few seconds, even for large files — most of the time is spent uploading.
Can I convert several MP4 files at once?
Yes. Drop up to 10 files at a time — each one is converted in parallel and you can download them individually or all at once.
Is there a file size limit?
Convert and edit files on your device. The browser edition accepts up to 100 MB total, media up to 10 minutes and video up to 4K. Codec availability and device memory can limit individual jobs.
Is this MP4 to MKV converter free and safe?
Yes. There are no pop-ups, redirects or fake download buttons, and no software to install. Files are transferred over HTTPS, processed in isolated job folders and deleted automatically.