CONVERT MKV → MP4 (MKV to MP4)
Convert Matroska video to MP4 for maximum device compatibility.
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HOW TO CONVERT MKV → MP4
Upload the MKV
Drop your .mkv file. We probe the video and audio codecs to plan the conversion.
Stream-copy or re-encode
If video is H.264/H.265 and audio is AAC, we copy streams (no quality loss); otherwise FFmpeg re-encodes.
Download the MP4
Get a cross-platform MP4, typically the same size as the source when stream-copied.
You almost certainly reached for this conversion because something refused to play your .mkv: an iPhone that greyed out the file, a Samsung TV that showed video but no sound, a Premiere import that threw an unsupported-format error, or an HTML5 video tag that stayed blank. Matroska is a richer container than MP4 — it can hold unlimited audio tracks, soft subtitles, chapters and attached fonts in one file — but that richness is exactly what trips up hardware decoders and Apple's ecosystem, which simply never shipped MKV demuxers. MP4 (the ISO base-media container) is the format those devices were built around, so moving into it is what gets the file playing. The video and audio inside your MKV are usually already H.264/H.265 with AAC or AC-3, so the picture you see does not change in any meaningful way — what changes is the wrapper around it and which extra tracks survive. Drop your MKV into the uploader above; KaijuConverter runs it through FFmpeg on our servers, hands back an .mp4, and deletes both files automatically. No account, no watermark.
Why convert MKV to MP4
The reason to leave MKV is compatibility, not quality. Matroska is the better engineering choice — it was designed to bundle several language tracks, soft subtitles you can toggle, chapter points and even subtitle fonts — but that flexibility is also why so many players reject it. iOS and iPadOS have no MKV demuxer, AirDrop and the Photos app will not touch it, many smart TVs and set-top boxes either skip the file or play video with no sound when the audio is DTS, and HTML5 video tags need MP4 to play in a browser without server-side transcoding. MP4 is the lowest common denominator that plays on phones, TVs, consoles, editors and the web, which is why the switch is worth the re-encode.
MKV vs MP4 — Technical specifications
Side-by-side comparison of the technical details.
MKV
- MIME type
- video/x-matroska
- Extensions
- .mkv, .mka (audio), .mks (subtitles)
- Container structure
- EBML (Extensible Binary Meta Language)
- Related
- WebM (restricted MKV subset)
- Max tracks
- Practically unlimited
- MIME Type
- video/x-matroska
- Extension
- .mkv
- Category
- Video Files
- Full Name
- Matroska Video
- Conversion Engine
- FFMPEG
MP4
- MIME type
- video/mp4
- MIME Type
- video/mp4
- Extension
- .mp4
- Category
- Video Files
- Full Name
- MP4 Video
- Conversion Engine
- FFMPEG
- Container
- ISO Base Media File Format (ISO/IEC 14496-12)
- Common video codecs
- H.264 (AVC), H.265 (HEVC), AV1, VP9
- Common audio codecs
- AAC, MP3, FLAC, Opus
- Max file size
- Practically ~16 TB; 2^63 bytes theoretical
- Streaming
- Supported with faststart (moov atom at front)
| Specification | MKV | MP4 |
|---|---|---|
| MIME type | video/x-matroska | video/mp4 |
| Extensions | .mkv, .mka (audio), .mks (subtitles) | — |
| Container structure | EBML (Extensible Binary Meta Language) | — |
| Related | WebM (restricted MKV subset) | — |
| Max tracks | Practically unlimited | — |
| MIME Type | video/x-matroska | video/mp4 |
| Extension | .mkv | .mp4 |
| Category | Video Files | Video Files |
| Full Name | Matroska Video | MP4 Video |
| Conversion Engine | FFMPEG | FFMPEG |
| Container | — | ISO Base Media File Format (ISO/IEC 14496-12) |
| Common video codecs | — | H.264 (AVC), H.265 (HEVC), AV1, VP9 |
| Common audio codecs | — | AAC, MP3, FLAC, Opus |
| Max file size | — | Practically ~16 TB; 2^63 bytes theoretical |
| Streaming | — | Supported with faststart (moov atom at front) |
MKV vs MP4 — Strengths and limitations
What each format does best, and where it falls short.
MKV Strengths
- Carries virtually any codec — H.264, H.265, AV1, VP9, Opus, FLAC, AAC, you name it.
- Multiple audio and subtitle tracks, chapters, and menus in one file.
- Patent-free container — no licensing fees.
- Attached fonts and metadata ride along for self-contained playback.
- Streamable and seekable with built-in index/cue tables.
Limitations
- Not natively supported in Apple's QuickTime or Safari without third-party tools.
- Windows needed codec packs (or "Films & TV" app updates) to play it out of the box.
- Hardware decoders on older TVs and streamers often reject MKV.
MP4 Strengths
- Universal playback — every browser, phone, TV, game console, and editing suite reads MP4.
- Supports modern codecs (H.264, H.265, AV1) with no container changes.
- Progressive streaming works with the "moov atom" at the start of the file.
- Carries subtitles, chapters, multiple audio tracks, and embedded metadata.
- ISO-standardized (ISO/IEC 14496-14) and patent-licensable via MPEG LA.
Limitations
- Codec licensing (H.264, H.265) carries royalty costs for commercial use.
- Streaming requires the moov atom at the start — a misplaced atom breaks web playback.
- Not ideal for lossless or professional editing workflows (use ProRes or DNxHD instead).
Quality & Compatibility
Be aware this conversion re-encodes the video — it is not a lossless rewrap. KaijuConverter runs FFmpeg with libx264 (H.264) at a near-transparent quality setting, so the loss is small and usually invisible at normal viewing distance, but it is a fresh encode, not a bit-for-bit copy of the original stream. File size therefore comes out close to the source rather than identical. On the track side, the consequences are concrete: MP4 keeps one video and the default audio track plus text subtitles converted to mov_text, but it cannot hold image-based subtitles (PGS, VobSub), ordered chapters, or multiple attached fonts — those are dropped. Container-level metadata and chapter markers may not carry across.
Tips for Best Results
- Test with a short clip first: trim 30 seconds of the MKV and convert that to confirm the audio track and subtitles you actually want are the ones that survive before processing a multi-gigabyte movie.
- If your MKV carries DTS or TrueHD audio (common on Blu-ray rips), expect the audio to be re-encoded to AAC for MP4 compatibility — that is usually what makes the file finally play with sound on a TV, but you lose the lossless audio track.
- Need the captions to stay? Make sure they are text-based (SRT or ASS) in the source, since bitmap PGS subtitles ripped from discs cannot be stored in MP4 and will simply vanish from the output.
Frequently Asked Questions
If the MKV contains embedded subtitle tracks, they will be included in the MP4 output as a separate subtitle stream when possible. Soft subtitles may be converted to a sidecar SRT file.
Only when it has to. If the codecs inside MKV (usually H.264 or H.265 for video, AAC for audio) are accepted by MP4, we stream-copy — the bytes are repackaged into the new container with zero re-encoding and no quality loss. When the source uses a codec the target does not support, we transcode at a matching bitrate to keep the visual quality close to the original.
No, if the source uses H.264/H.265 video and AAC audio. We stream-copy those tracks bit-for-bit. Re-encoding only happens for uncommon codecs and uses near-transparent settings.
If the MKV already contains H.264 video and AAC audio, KaijuConverter can remux (copy) the streams without re-encoding, which is instant and lossless. Re-encoding occurs only when the codecs are incompatible.
With stream copy, expect the job to finish in seconds to tens of seconds regardless of video length — the work is mostly rewriting the container. Transcoding is slower (roughly real-time: a ten-minute clip takes about ten minutes) because every frame must be decoded and re-encoded. The progress bar shows which mode applies.
iOS does not natively support the MKV container, even if the codec inside is compatible. Repackaging to MP4 fixes the rejection without touching the video.
Free users can convert files up to 25 MB free (up to 2 GB with Kaiju Cloud). For full-length movies and high-resolution video, premium plans support files up to 2 GB.
Yes. Resolution, frame rate, colour space and bit depth are preserved by default; stream copy is literally bit-identical on these parameters. If you explicitly pick a lower bitrate or a different codec in Advanced, the output is rebuilt to those settings, but the default is always "match the source".
Text-based subtitles (SRT, ASS) are converted to MP4-compatible mov_text. Bitmap subtitles (PGS, VobSub) cannot be stored in MP4 and are dropped.
Dramatically — stream-copy is limited by disk speed (often sub-minute for a 2 GB file). Re-encoding is CPU-bound and can take many minutes.
Matroska Video
Source formatMKV is a flexible, open-standard container format that can hold unlimited video, audio, subtitle, and metadata tracks. It is popular for high-definition video and supports virtually any codec.
MP4 Video
Target formatMP4 is the most universally supported video container format. It typically uses H.264 or H.265 video codecs with AAC audio, providing an excellent balance of quality and file size across all devices and platforms.
Going the other way? Convert MP4 back to MKV with the same engine.
Common Use Cases
iPhone and Apple TV playback
Apple ecosystem refuses MKV; MP4 plays in Photos, AirDrop, and native TV apps.
Web video embeds
HTML5 <video> supports MP4 universally; MKV requires server-side transcoding.
Smart TV and Chromecast
Many Smart TVs skip MKV or drop audio; MP4 is the reliable choice.
Video editing import
Premiere, Final Cut, and DaVinci Resolve prefer MP4 for smooth scrubbing.
MKV vs MP4 — Typical file sizes
Approximate file sizes for common scenarios.
- 45-min episode (H.264 1080p) 800 MB - 1.6 GB
- 2-hour movie (H.265 1080p) 1.5-3 GB
- 2-hour movie (4K HDR H.265) 15-40 GB
- Smartphone video (1080p, 1 min) 60–120 MB
- 4K video (1 min, H.265) 200–400 MB
- Streamed movie (90 min, H.264) 1–4 GB
Working with 4K/8K Video or Files Over 100MB?
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10-bit color grading workflows, Apple ProRes mastering, and automated watch-folder queues.
Related comparisons
See these formats side by side to understand which fits your use case best.
Related Guides
MKV/Matroska Container Format: The Open-Source Multimedia Powerhouse
Complete guide to MKV/Matroska container format: EBML architecture, unlimited codec support, chapter systems, PGS/ASS subtitles, mkvtoolnix commands, and...
Read guide →MP4 Container Format: The Universal Video Standard
Deep dive into MP4 container format: ISOBMFF box structure, fMP4 streaming, fast-start optimization, codec compatibility, and ffmpeg encoding commands.
Read guide →MKV (Matroska) Video Container: Complete Format Guide
Everything about MKV video containers — how Matroska works, supported codecs, subtitle tracks, chapters, and how to convert MKV files.
Read guide →
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