User Guide
Add your MKV file
Drop the .mkv in or click to browse. Files up to 300 MB are accepted. The engine downloads once (~30 MB) and is cached for every later conversion.
Check what is inside first
If your MKV has more than one audio language or embedded subtitles, know that only the first video and first audio track are carried across. Subtitles are not retained. Extract them separately if you need them.
Press Convert
Video is encoded to H.264 with the veryfast preset and audio to AAC. MKV frequently holds H.265, VP9 or AV1, none of which MP4 players reliably accept, so a full re-encode is the only dependable route.
Allow time for HD sources
MKV is the usual container for 1080p and 4K rips, and those are the slowest cases. A 10 minute 1080p file can take 15 minutes or more. Leave the tab in the foreground.
Download and verify
Play the first few seconds and skip to the end before deleting the original — that confirms both the audio sync and that the file finished writing.
If it runs out of memory
4K MKV files routinely exceed the browser’s budget. Cut the section you need with the Video Trimmer first, or reduce it with the Video Compressor.
About the MKV to MP4 Converter
Matroska (.mkv) is, on the technical merits, a better container than MP4. It is open, it is extensible, it stores unlimited tracks, chapters, attachments and subtitle formats, and it recovers gracefully from truncation. It also plays almost nowhere outside VLC. This tool converts it into MP4 — technically the weaker container, practically the only one that works everywhere.
The compatibility gap
No version of Safari plays MKV. Neither does QuickTime, iOS, most smart TVs, PowerPoint, or Premiere Pro without a plug-in. Chrome and Firefox will sometimes play the video track and stay silent, because they support the container poorly and the codecs inside it selectively. The MDN media container guide documents this directly: MKV is not a format browsers are expected to handle.
MP4 has the opposite profile. As the MDN video codec reference notes, H.264 in MP4 is the one combination with effectively universal decoder support — hardware accelerated on virtually every phone, laptop and TV sold in the last decade.
What gets lost — read this before converting
MKV’s strength is that it holds many things at once. MP4 conversion here keeps two of them. This is the honest limitation of the tool and the most common reason someone is disappointed by the result:
| What your MKV may contain | Survives the conversion? |
|---|---|
| First video track | Yes — re-encoded to H.264 |
| First audio track | Yes — re-encoded to AAC |
| Additional audio languages | No |
| Embedded subtitles (SRT, ASS, PGS) | No |
| Chapter markers | No |
| Attachments (fonts, cover art) | No |
If you need the subtitles, extract them to a .srt file with a desktop tool before converting, then load them alongside the MP4 in your player. The Matroska element specification documents how these elements are stored if you want to understand what is in your file.
Why it always re-encodes
A container change alone — a remux — is instant and lossless, but only works when the codecs inside are already MP4-legal. MKV files usually fail that test. They commonly carry H.265/HEVC, VP9 or AV1 for video, and FLAC, Opus, DTS or AC-3 for audio. Several of those cannot legally sit in an MP4 at all, and the ones that can are unevenly supported by players.
So this tool re-encodes unconditionally: H.264 video at CRF 23 with the veryfast preset, AAC audio. You trade one generation of quality for a file that genuinely plays anywhere. The FFmpeg H.264 encoding guide explains the CRF scale in detail — lower is better quality and a bigger file, and 23 is the default because it is the point at which most viewers stop seeing artefacts.
Speed, size and limits
Everything runs through a WebAssembly build of FFmpeg that is downloaded into the browser tab on first use (about 30 MB, cached afterwards). The video itself is never transmitted — there is no upload step, no server-side queue and nothing to delete afterwards. Because a single-threaded WebAssembly module is capped at roughly 2 GB of addressable memory, files above 300 MB are rejected outright and anything over 60 MB shows a slow-work warning before it starts.
MKV is the container of choice for high-resolution video, which makes it the format most likely to hit those ceilings. As a rough guide: 300 MB is about 20 minutes of 1080p at a typical 2 Mbps, or under 5 minutes of 4K. Expect encoding to take longer than the clip runs — 1080p on a mid-range laptop runs at roughly half real time, so a 10 minute video takes about 20 minutes.
Frequently Asked Questions
Why can my computer play MKV but my phone cannot?
Your computer almost certainly has VLC or a codec pack installed. Phones, browsers and TVs ship only the codecs they license — and MKV support is not among them. Safari and iOS have never supported MKV in any version. Converting to MP4 removes the dependency entirely.
Will my subtitles be kept?
No. Only the first video track and first audio track are carried over. Embedded subtitles, chapters and any extra audio languages are dropped. If subtitles matter, extract them to a .srt file with a desktop tool before converting and load them alongside the MP4.
Why does it re-encode instead of just changing the container?
Because a straight rewrap only works if the codecs are already MP4-legal, and MKV files usually carry H.265, VP9, AV1, FLAC, Opus or DTS — some of which cannot sit in an MP4 at all. Re-encoding to H.264 and AAC is the only route that reliably plays everywhere.
Is any quality lost?
One generation, yes. The tool encodes at CRF 23, FFmpeg’s default, which is intended to be visually transparent. You are unlikely to see the difference in normal viewing, but it is not mathematically identical to the source.
How long does a 1080p file take?
Roughly twice its runtime on a mid-range laptop — a 10 minute clip takes about 20 minutes. WebAssembly runs at 50–80% of native speed and this build uses a single CPU core, so it cannot match desktop FFmpeg.
My 4K MKV will not convert. What can I do?
4K files usually exceed the 300 MB cap and the browser memory budget. Trim to the section you actually need with the Video Trimmer, or run it through the Video Compressor at 1080p first, then convert.
Is the file uploaded anywhere?
No. FFmpeg runs as WebAssembly inside your browser tab. The video is read into memory locally and written back to your downloads folder. Nothing is transmitted at any point.
Which is actually the better format?
MKV, on the technical merits — it is open, extensible and holds unlimited tracks, chapters and subtitles. MP4 is the better format in practice, because it is the only one Safari, iOS, smart TVs, PowerPoint and every editor will open without help.