Convert MKV to MP4
To convert MKV to MP4, add your .mkv file below: ffmpeg in your browser re-encodes it to H.264 video with AAC audio, which plays on phones, TVs and browsers that won't open MKV. Nothing is uploaded.
About converting MKV to MP4
MKV (.mkv) files can be converted to MP4 entirely in your browser — no software to install, no file upload, and no account required. The conversion runs on a WebAssembly build of ffmpeg directly on your device.
Why convert MKV
MKV (Matroska) is a flexible container popular for recordings and downloads, but many phones, smart TVs, game consoles and editing apps only accept MP4.
What's kept and what isn't
If the MKV already holds H.264 video and AAC audio, desktop ffmpeg can just switch the container without any quality loss: ffmpeg -i input.mkv -c copy output.mp4. To pick specific tracks, add -map options on desktop.
- One video and one audio track are kept (ffmpeg's default choice: the highest-resolution video and the audio track with the most channels)
- Extra audio languages aren't carried over; image-based subtitles (PGS/VobSub) can't be stored in MP4
- Video: H.264 at CRF 23, "veryfast" preset; audio: AAC — a lossy re-encode that looks very close to the source
Technical comparison
| MKV | MP4 | |
|---|---|---|
| Category | video | video |
| Compression type | Varies (codec-dependent — a container, not a codec) | Lossy (H.264/H.265 video, AAC audio) |
| Typical file size | Varies — depends entirely on the codecs it wraps | Moderate — a few MB per minute of 1080p video, varying with CRF/bitrate |
| Transparency support | Depends on the wrapped codec | No |
How to convert MKV to MP4 privately
Every step below runs on a WebAssembly build of ffmpeg inside this browser tab. Your .mkv file is never uploaded to a server — there's no network request carrying it anywhere.
Add your MKV file
Drag and drop, or click to select, a file (.mkv). It's read straight from disk into browser memory — no upload step exists.
Convert locally
FreePrivateConvert converts it to MP4 using a WebAssembly build of ffmpeg, entirely within this browser tab.
Download the result
Save your converted .mp4 file. It only ever existed in this tab's memory — closing the tab without downloading discards it completely.
Troubleshooting
- Conversion is slow
- Encoding runs on one CPU core in the browser; long or high-resolution files can take many minutes. Keep the tab in the foreground.
- The wrong audio language came through
- ffmpeg picks one audio track automatically. Use the desktop command with -map to choose a different one.
Convert MKV to MP4 with ffmpeg (command line)
Prefer the terminal? This is the exact ffmpeg command this page runs for you:
ffmpeg -i input.mkv -c:v libx264 -preset veryfast -crf 23 -c:a aac output.mp4- -i input.mkv
- Your MKV file.
- -c:v libx264
- Encode the video as H.264, the format virtually every phone, browser and TV plays.
- -preset veryfast
- Encoding speed vs. file size trade-off: veryfast encodes quickly with a slightly larger file.
- -crf 23
- Constant quality level (lower = better quality and bigger file; 18-28 is the usual range, 23 is the default).
- -c:a aac
- Encode the audio as AAC.
- output.mp4
- The MP4 file to create. ffmpeg picks the container from this extension.
Don't have ffmpeg installed? You don't need it: use the converter at the top of this page. It runs the same conversion on your own device, and your file is never uploaded.
FAQ
- Is my file uploaded anywhere?
- No. Your file is read and processed inside this browser tab and never leaves your device. You can confirm it in your browser's developer tools: the Network tab shows no upload.
- Does converting MKV to MP4 reduce quality?
- The video is re-encoded as H.264 at CRF 23 (ffmpeg's default quality level, "veryfast" preset) with AAC audio. That looks very close to the source for most footage, but it is a lossy re-encode, so fine detail and grain can soften slightly.
- Is there a file size limit?
- Yes: 500MB per file. There's no account and no limit on how many files you convert.
- What's the difference between MKV and MP4?
- MKV (Matroska Video) and MP4 (MPEG-4 Part 14) store data differently. MKV: varies (codec-dependent — a container, not a codec). MP4: lossy (H.264/H.265 video, AAC audio). MP4 is the default video container for the web and virtually every device — pairs H.264/H.265 video with AAC audio in a single file.
- Why convert MKV to MP4?
- Common uses for MP4: Uploading to YouTube and social platforms; Sharing screen recordings; Web <video> embeds. Near-universal hardware decoding support across phones, browsers, TVs, and game consoles, so playback rarely falls back to slower software decoding.
- What MIME type does a .mp4 file use?
- video/mp4, the standard type for MPEG-4 Part 14.
- What is the ffmpeg command to convert MKV to MP4?
- Run: ffmpeg -i input.mkv -c:v libx264 -preset veryfast -crf 23 -c:a aac output.mp4. This is the exact command FreePrivateConvert runs for this conversion, so you can use this page instead of installing ffmpeg: it runs the same conversion inside your browser.
Learn more: What is an MKV file
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