Compress · runs on your device

Compress MKV files in your browser

Add your .mkv files, set a target size or quality level, and Video Mender re-encodes them locally — keeping the MKV container by default, or writing MP4 if you need broader playback.

Nothing is uploaded

Drop video files here

MKV · WebM · MP4 · MOV

Choose video files to convert

Output format

VLC and MPV play it anywhere; browsers, Safari, iOS and most TVs do not.

How to size the output

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

Quality — 6 of 10

Resolution limit

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

Frame rate

Audio

Platform presets — editable

These are dated observations, not permanent platform limits — all were last checked on 2026-08-01. Every one sets an ordinary target size you can edit afterwards.

MKV files are the ones people most often need to shrink, because they are the ones that arrive at 8, 20 or 40 gigabytes. The container is not the reason: MKV adds almost no overhead. The size comes from what is inside, typically high-bitrate H.264 or HEVC at 1080p or 4K, sometimes with several audio tracks.

That means compressing an MKV is a straightforward bitrate decision, and the biggest lever is usually resolution rather than bitrate. A 4K source destined for a laptop screen should be capped at 1080p first — that alone removes about three quarters of the pixels, and the result looks better than the same file size achieved by starving 4K of bits.

MKV is kept as the output container by default, because it is a good archival format and because you asked for MKV compression rather than conversion. MP4 is one control away when you need something that plays in a browser or on a TV. Note that only the first video and first audio track survive either way: extra language tracks, subtitles and chapters are dropped.

What this actually does to your file

Rewrap or re-encode

Rewrapping an MKV changes nothing about its size, since MKV overhead is already negligible. Compression here always re-encodes the video, which is the only thing that reduces the bitrate.

Quality

A lossy re-encode. Because MKV sources are usually high bitrate, the Balanced preset at a capped resolution typically achieves a large reduction with little visible difference — but this stacks a generation of loss on whatever the source already had.

Only the first video and first audio track are kept. Multiple language tracks, subtitle streams and chapter markers are dropped.
Large MKV files are the most likely to exhaust device memory. The queue estimates memory first and processes one heavy file at a time where necessary.
4K re-encoding in WebAssembly is slow. Capping to 1080p is usually both faster and the better-looking result.
If the MKV holds HEVC and you choose MP4 output with H.264, the file grows relative to HEVC at equal quality — that is the codec difference.

How to use this page

Add your .mkv files. Very large files are expected here.
Cap the resolution first if the source is 4K and the destination is not.
Choose a quality preset, or set a target size in megabytes.
Start the queue and watch real per-file progress; large files take real time.
Download each output as it completes rather than accumulating several in memory.

Where the result plays

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

Target	Works	Detail
MKV output (default)	Partly	VLC and MPV play it anywhere. No browser, no Safari, no phone gallery.
MP4 output	Yes	H.264 and AAC. The choice for browsers, phones, TVs and editors.
Plex and media servers	Yes	Both containers work, but MP4 avoids server-side transcoding on playback.
Smart TVs	Partly	MKV support varies by model. MP4 with H.264 is the dependable option.

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.
WebM	Yes	Yes	A deliberately restricted Matroska profile for the web: VP8/VP9/AV1 video with Opus or Vorbis audio, and nothing else.
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.

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 browser ran out of memory on a large MKV.

Cap the resolution, close other tabs, and download each result before starting the next. The limits page lists the file sizes and resolutions that have actually been tested.

My other audio languages disappeared.

Only the first audio track is kept. Choose the language you need before converting, or use a desktop editor when the output must preserve multiple audio tracks.

Compression is extremely slow on a 4K file.

Expected — 4K re-encoding in WebAssembly is heavy. Cap the resolution at 1080p, which is faster and usually looks better at any given file size.

The output is still very large.

A long high-resolution film has a floor below which it cannot go while staying watchable. Use target-size mode to state your requirement explicitly and see the estimated quality cost before starting.

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

How do I compress an MKV file?

Re-encode the video at a lower bitrate, and cap the resolution if the source is larger than the destination needs. Add the file here, choose a target size or quality preset, and it runs in your browser without uploading.

Why are MKV files so large?

Not because of the container — MKV overhead is minimal. It is what is inside: high-bitrate 1080p or 4K video, often with several audio tracks. Reducing the video bitrate or resolution is what shrinks the file.

Can I compress an MKV without losing quality?

No, and rewrapping will not help either, since MKV adds almost no overhead to remove. What is realistic is loss you cannot see — usually achieved by capping the resolution rather than starving the bitrate.

Should I compress to MKV or MP4?

MKV for archiving and for local players like VLC. MP4 if the file needs to play in a browser, on a phone, on a TV or in an editor. Both are one control apart here.

Is there a size limit on the MKV I can compress?

Yes. The browser engine is 32-bit WebAssembly with a 4 GiB address space, which caps a re-encode at roughly 1 GB of source on a desktop. Large films may need splitting first. The queue estimates memory and refuses rather than crashing; every figure is on the limits page.