Free converter · Remux (container swap)

Convert MKV to MP4

Runs in your browserNo uploadUp to 100 MB

Short answer

Drop your MKV file into the converter below and download the MP4 file. It runs in your browser with FFmpeg, for files up to 100 MB; nothing is uploaded. The video and audio streams are copied into the MP4 container without re-encoding, so quality and file size stay the same. If a stream cannot go into MP4 (PCM audio cannot go into MP4, for example), the converter re-encodes to H.264 video and AAC audio in MP4, at FFmpeg's default quality, with the source's sample rate instead.

Choose fileVIDEO/X-MATROSKA, MKV · 100 MB maximumChoose a .mkv file - it stays on your device

Maximum file size: 100 MB. The first conversion loads a ~30 MB converter engine, once per visit. Nothing is uploaded - conversion runs on your device.

Streams are copied; MKV-only codecs or tracks are re-encoded or left out. · Remux (container swap) · Browser-side via FFmpeg WASM - nothing uploaded.

Key takeaways

H.264 and AAC inside an MKV are copied into MP4 without re-encoding: the stream hashes matched in our test.
The converter maps video and audio streams only, so subtitle tracks and attachments in the MKV are not in the MP4.
MKV can hold almost any codec; MP4 cannot. A copy that fails falls back to a re-encode.
The MP4 was 7,797 bytes (0.085%) larger than the MKV in our test, from the index data.

Measured: the same streams in a different file

Measured on a 10 second, 1920 × 1080, 30 fps excerpt of Big Buck Bunny (animated film, not camera footage; (c) Blender Foundation, CC BY 3.0) as H.264 plus stereo AAC 192 kbps, using the same ffmpeg arguments this converter passes to its WebAssembly build, run with ffmpeg 6.1.1. Results describe this file, not every file.

Check	Result
Video stream hash	Identical to the MKV's
Audio stream hash	Identical to the MKV's
Size	9,175,114 bytes to 9,182,911 bytes (+7,797, +0.085%)
Video in the MP4	H.264, yuv420p, 1920 × 1080, 30 fps
Audio in the MP4	AAC, 48 kHz, stereo
Time, native ffmpeg	0.10 s

MKV vs MP4

	MKV	MP4
Full name	Matroska	MPEG-4 Part 14
What is inside	Almost any codec, plus several audio and subtitle tracks	H.264 or H.265 video with AAC audio
Compression	Depends on the codec inside	Depends on the codec inside
Best for	Archiving and playback in desktop players; most social platforms do not list it	Uploading anywhere: the format social platforms document first

What you get

The same streams in an MP4 file. When a stream cannot be copied, the converter writes H.264 video and AAC audio in MP4, at FFmpeg's default quality, with the source's sample rate.

What an MKV can hold that this conversion drops

The converter uses -map 0:v? -map 0:a?, which selects every video and every audio stream and nothing else. FFmpeg's documentation states that when you give -map explicitly, no automatic selection happens, so subtitles, attachments and chapters-as-tracks are not copied. MKV files often carry soft subtitles; if you need them in the MP4, add a subtitle map and a text-compatible codec yourself: ffmpeg -i in.mkv -map 0 -c copy -c:s mov_text out.mp4, which works for text subtitles only.

Codecs that MKV allows and MP4 does not

PCM audio cannot be copied into MP4 (the converter's own code names it as the case that fails); the converter then re-encodes the file to H.264 and AAC.
Image-based subtitles (PGS, VobSub) cannot be converted to mov_text.
Run ffprobe -hide_banner in.mkv and read the Stream lines before you convert a long file.

For uploads, YouTube recommends MP4 with H.264 and AAC-LC or Opus. LinkedIn lists MKV among supported file types; TikTok's API and Pinterest do not, so MP4 is the common denominator.

Questions people ask

Is this MKV to MP4 converter free and private?

Yes. It is free, needs no account and runs entirely in your browser, so the file never leaves your device.

Will converting MKV to MP4 reduce quality?

Not when the streams fit MP4: they are copied unchanged. Only if a stream has to be re-encoded is there a small, usually invisible loss.

Is there a file size limit?

Files up to 100 MB. The first conversion downloads a converter engine of about 30 MB once per visit; larger files are better handled by a desktop app such as HandBrake or FFmpeg.

Does MKV to MP4 lose quality?

Not when the streams are copied. In our test the video and audio stream hashes were identical after conversion.

Where did my subtitles go?

The converter copies video and audio streams only. Subtitle tracks stay behind; use ffmpeg with -map 0 and -c:s mov_text for text subtitles.

Why does my MKV take long to convert?

A copy is almost instant. A long conversion means the converter had to re-encode because a stream could not go into MP4.

Will the MP4 play everywhere?

It will if the codecs inside are supported by the player. Check them with ffprobe.

More converters

Sources and how we checked

MDN: Media container formats
MDN: Web audio codec guide
MDN: Image file type and format guide
FFmpeg documentation: stream copy
FFmpeg Documentation: ffmpeg (stream selection, -map, -c copy) (read 2026-10-08)
FFmpeg Formats Documentation: MOV/MP4 muxer (+faststart) (read 2026-10-08)
YouTube Help: Recommended upload encoding settings (read 2026-10-08)
LinkedIn Help: Video sharing troubleshooting (supported file types) (read 2026-10-08)
Blender Foundation: Big Buck Bunny (CC BY 3.0), the clip used for the measurements (read 2026-10-08)

Reviewed October 8, 2026. What the converter writes was measured with ffprobe on the same FFmpeg build your browser downloads; format descriptions follow MDN's media guides.