Convert · runs on your device

Convert MKV to MP4 in your browser

Drop your MKV files in and Video Mender rewraps them as MP4 — usually without re-encoding a single frame, because the video inside an MKV is normally already H.264 that MP4 can carry as-is.

Nothing is uploaded

Drop video files here

MKV

Choose video files to convert

Output format

Plays on effectively every browser, phone, TV and editor when it holds H.264 and AAC.

Video codec

The universal baseline. Hardware decoding exists on essentially every device sold since 2010.

How to size the output

A constant quality target. File sizes vary with how demanding the footage is.

Quality — 8 of 10

Resolution limit

Files smaller than the limit are left alone. Capping usually looks better than lowering the bitrate.

Frame rate

Audio

Fast rewrapCopy streams without re-encoding when the codecs already fit

Lossless and near-instant when it applies. It cannot reduce file size, so turn it off if you need a smaller file.

MKV is a container, not a codec. A typical MKV holds H.264 video and AAC or AC-3 audio, and MP4 can hold H.264 and AAC too. That means the conversion is often a rewrap: the same compressed frames are copied into a new box structure, byte for byte, and finish in seconds regardless of how long the video is.

Video Mender checks each file first and tells you which path it will take before anything runs. If the streams are compatible you get "Fast rewrap — no quality loss". If the MKV holds something MP4 cannot carry, such as VP9 video or a FLAC audio track, the file is re-encoded instead and the queue says so plainly.

Everything happens inside this tab. Your MKV never reaches a server, which matters when the file is a film rip that would take a long time to upload and might not be yours to hand to a third party in the first place.

What this actually does to your file

Rewrap or re-encode

Rewrap is attempted whenever the MKV holds H.264, HEVC or MPEG-4 video with AAC or MP3 audio. VP8, VP9, Theora, FLAC and Vorbis have to be re-encoded because MP4 has no standard way to carry them. AC-3 is a special case: MP4 permits it but QuickTime and Safari play it silently, so the video is still copied while the audio alone becomes AAC.

Quality

When the rewrap path runs, quality is mathematically identical — no frame is decoded or re-encoded. When re-encoding is required, some loss is unavoidable; the quality slider decides how much.

MKV can carry several audio tracks and embedded subtitles. This converter keeps the first video and first audio track; extra tracks and subtitle streams are dropped.
Chapter markers are not carried across to MP4.
A rewrapped MP4 is the same size as the MKV. If you need it smaller, that is compression, not conversion — use the video compressor.

How to use this page

Drop one or many MKV files onto the queue, or pick a whole folder.
Check the badge on each file: "Fast rewrap" means no re-encode, "Re-encode" means the codec had to change.
Leave the output as MP4 and press Start.
Download files individually as they finish, or select several and take a ZIP.

Where the result plays

Checked against the containers and codecs this page produces, stated conservatively.

Target	Works	Detail
Safari and iOS	Yes	MP4 with H.264 plays natively; MKV does not play at all.
Chrome, Edge, Firefox	Yes	MP4 plays inline; MKV downloads instead of playing.
Premiere Pro, Final Cut, Resolve	Yes	MP4 imports directly. MKV needs a plugin or a prior conversion.
Smart TVs and consoles	Yes	MP4/H.264 is the most widely accepted combination on TV hardware.

The formats involved

Format	Read	Write	What it is
MKV	Yes	Yes	Matroska. The most permissive common container: it will carry almost any codec, plus multiple audio tracks, chapters and subtitles.
MP4	Yes	Yes	The MPEG-4 Part 14 container. It is a wrapper, not a codec — an MP4 usually holds H.264 or HEVC video with AAC audio.
MOV	Yes	Yes	Apple QuickTime. Structurally a close cousin of MP4, which is why MOV files can often be rewrapped to MP4 with no re-encode.
WebM	Yes	Yes	A deliberately restricted Matroska profile for the web: VP8/VP9/AV1 video with Opus or Vorbis audio, and nothing else.

Tested limits

Real ceilings measured on real devices, not a marketing claim of unlimited.

Limit	Desktop	iPhone / iPad	Why
File size — rewrap	1.3 GiB	384 MiB	Rewrapping never decodes a frame, so it tolerates far larger files.
File size — re-encode	1.0 GiB	256 MiB	Decoding holds uncompressed frames in memory, which is the real constraint.
Clip length — re-encode	4:00:00	30:00	Beyond this, progress reporting and memory behaviour stop being dependable.
Resolution — re-encode	4096 × 2160	1920 × 1080	A 4K frame is about 12 MB decoded, before the encoder allocates anything.

If something goes wrong

The MP4 has no sound.

The MKV almost certainly used an audio codec MP4 cannot carry, such as FLAC or DTS. Set the audio output to AAC to force a re-encode of the audio track while the video is still copied.

My subtitles are gone.

MKV subtitle tracks are not transferred. Extract them with a dedicated tool first if you need them; burning subtitles into the picture is out of scope here.

The file failed with "unsupported video codec".

A small number of MKV files hold codecs this build cannot decode at all, such as some AV1 profiles or lossless FFV1. The queue marks these unsupported rather than producing a broken file.

Your video never leaves this device

This is a structural property of the application, not a policy we ask you to take on trust. There is no upload endpoint, no API route and no server action anywhere in the codebase, so there is nowhere for a video to be sent even by mistake.

Conversion runs in WebAssembly inside this browser tab. The engine is served from this domain and loaded only when you start a job.
The Content-Security-Policy restricts network connections to this origin, so the page is not permitted to contact a third party at all.
Nothing is stored. Close the tab and every queued file and finished output is gone — which is also why you should download results before you leave.
There are no accounts and no AI. Anonymous page views are counted — which page, roughly what country, what kind of device — and nothing whatsoever about your files.

The repository contains a test that fails the build if any media byte is passed to fetch, XMLHttpRequest, sendBeacon or a WebSocket. See the privacy page for how that is verified.

Questions

Does converting MKV to MP4 lose quality?

Not on the rewrap path, which is what most MKV files take. The compressed video stream is copied unchanged into the new container, so the result is pixel-identical to the source. Quality is only affected when the codec inside has to change.

Why is a large MKV rewrapped in seconds?

Because nothing is being re-compressed. Rewrapping only rewrites the container metadata and copies the existing frames, so it runs at disk speed rather than encoding speed.

Is there a file size limit?

Yes, and it is published rather than vague. The browser engine is a 32-bit WebAssembly program with a 4 GiB address space that has to hold the source and the output at once, so rewrapping tops out around 1.25 GB and re-encoding around 1 GB on a desktop, less on a phone. The limits page lists every figure.

Do you upload my video anywhere?

No. The conversion runs in WebAssembly inside this browser tab. There is no upload endpoint in this application, and the site sends no media bytes over the network — a test in the repository asserts it.

Can I convert a whole folder of MKV files at once?

Yes. Queue as many as you like and Video Mender processes them with a memory-aware scheduler, showing per-file and overall progress. On phones it deliberately runs one heavy file at a time.