Convert MKV to MP4
MKV is Matroska - a video file wrapper that download sites, anime scene releases, and archival tools tend to ship in because it can hold basically anything. The trouble: iPhones, iPads and Macs won't open it, most web players don't accept it, and most upload forms and social apps reject it. (Windows 10 and 11 are the exception - their built-in players handle MKV, as long as the formats inside are ones Windows supports.) When what's inside your MKV is already MP4-friendly (which is most files - H.264 or HEVC video with AAC audio), Wave just changes the wrapper. No re-encoding, no quality loss, no upload. Runs entirely in your browser tab, and finishes in seconds.
What actually changes
To understand what Wave is doing, it helps to know that a video file is really two things stacked together: the container (the wrapper, .mkv or .mp4) and the codec (the compression format for the actual picture and sound). Most MKVs contain H.264 or HEVC video with AAC audio - and MP4 accepts both of those codecs as they are. That means the conversion doesn't need to touch the video or audio at all. It just rewrites the outer wrapper. (When the audio is AC-3, DTS, FLAC or Opus instead, Wave still copies the video untouched and converts only the sound to AAC, which takes a few extra seconds.) The technical term is "remux" (short for re-multiplexing), and it takes seconds regardless of file size because nothing is being re-processed. If the codec vs. container distinction is new, Codecs vs containers walks through the whole thing in plain English.
One extra thing Wave does on the way out: it moves the moov atom - MP4's tiny map that tells players where every video frame lives in the file - to the front. Without this, a mobile browser or cloud preview would have to buffer the whole file before it could start playing. With the moov atom at the front, playback starts instantly. Wave does this on every MP4 it writes; you don't have to think about it.
The one case where a plain container swap won't work: some MKVs carry codecs MP4 can't hold - VP9 video and Vorbis audio are the usual culprits (both open-source codecs that YouTube and browser recording tools produce). In that case Wave has to actually decode and re-encode the offending stream. That takes longer (measured in minutes on a modern laptop, versus seconds for a plain remux) and adds a tiny amount of compression loss - usually well below what any human can see. See Remux vs re-encode for what changes on the slower path. All Wave's encoders are LGPL-licensed (meaning nothing about them contaminates other software you might attach), never GPL x264 - so no licensing surprises land in the output.
A short history for context: Matroska (MKV) was designed by an open-source team starting in 2002 to be an unrestricted container - one that could hold every video and audio codec that would ever exist. There's no limit on frame rate, no ceiling on how many audio or subtitle tracks a file can carry, and the format has native support for embedded fonts (used by styled subtitles) and even a design that survives partial file corruption better than MP4 does. Those design choices are why MKV became the default for high-quality video archival, anime encodes, and downloaded rips. They're also why iOS refuses to touch it: MKV's file format has a much larger surface area than MP4's, which makes it harder to parse safely on a resource-constrained device.
For everyday MKVs - one video track, one audio track, maybe subtitles - Wave's remux takes a few seconds and produces a file that's identical in every measurable way to the source. If your MKV has multiple audio tracks (director's commentary, dubbed audio, alternate language), Wave keeps all of them, and chapter markers come along too. Text-based subtitles (SubRip .srt, ASS/SSA, WebVTT) get converted into MP4's equivalent format (mov_text) and travel through. Bitmap subtitles (PGS from Blu-ray rips, VobSub from DVD rips) are dropped - MP4 has no container-level home for them, so there's nowhere to put them. If subtitles matter and yours are bitmap-based, keep the MKV alongside the MP4. One more reason MKVs fail on TVs: DTS surround audio, which Samsung and LG have dropped from many models. Wave converts DTS to AAC while copying the video - see Why your TV won't play videos from USB and MP4 has no sound.
Three steps.
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Step 1
Drop your MKV file onto the card, or click "Pick a video".
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Step 2
Wave inspects the source and shows whether it can fast-copy or needs to re-encode.
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Step 3
Hit convert. Save the MP4 when it finishes.
Answers, specifically.
Will converting MKV to MP4 reduce quality?
In the common case, no - not by any amount you'd see, hear, or measure. Wave copies the compressed video and audio bytes through the conversion unchanged when they're already in a format MP4 accepts (H.264 or HEVC video paired with AAC or MP3 audio, which is what nearly every MKV in the wild contains). The output file is bit-identical to the source in every way that matters for playback. The rare exception is when the MKV holds a codec MP4 can't carry (VP9 video or Vorbis audio - both open-source formats used by YouTube downloads and some browser recordings), in which case Wave does a real re-encode. Even then the loss is usually well below the threshold anyone can perceive on typical viewing gear.
Can iPhone / iPad / QuickTime play MKV?
No - and this is the main reason the conversion exists. Apple has never added MKV support to iOS or macOS, and there's no sign it will. Windows 10 and 11 do play MKV in their built-in apps, and VLC plays it everywhere, but VLC is an install most non-technical viewers don't have. Converting to MP4 fixes the Apple side: an MP4 with H.264 video and AAC audio plays natively on every iPhone, iPad, Mac, Windows PC, Android device and web browser, with nothing for the viewer to install. If the video inside is HEVC, it still plays on Apple devices and most Android phones, but Windows needs the HEVC Video Extensions for it.
Does my MKV file get uploaded anywhere?
No, not a single byte. Wave runs the entire conversion inside your browser tab using a technology called WebAssembly (essentially a way for browsers to run heavy computation locally, at close to native speed). The file you drop in never crosses the network. If you want to verify this - open your browser's developer tools, go to the Network tab, and watch while a conversion runs. You'll see no upload activity for your file. This design is what makes Wave different from every cloud converter: there's no server holding your video, so there's no server that could leak it, log it, or "delete it later".
How large a file can I convert?
Up to about 8 GB comfortably in a modern desktop browser. The real ceiling is your browser's memory: Wave streams the file in as it works, but the encoder still needs working memory that scales with resolution and complexity. Above 8 GB (which is unusual for anything shorter than a Blu-ray remux) Wave refuses the file with a clear message; above 2 GB Wave warns but lets you proceed. Files under 2 GB just work; that covers the vast majority of MKV downloads people convert.
Why is the MP4 sometimes slightly larger than the MKV?
MKV packs its metadata (the file's bookkeeping - timestamps, chapter markers, subtitle info, and so on) a little more efficiently than MP4 does. On a remuxed conversion, the video and audio bytes are identical between the two files - so the size difference is entirely in the wrapper overhead. Expect the MP4 to be within one or two percent of the MKV's size. If the MP4 is dramatically smaller, that's a sign the source had extra streams (multiple audio tracks, embedded fonts, or attachments) that MP4 couldn't carry through.
Which audio track does Wave pick if the MKV has several?
The default-flagged audio track, which the container marks explicitly. Where no default is set, Wave picks the first audio stream in the file. Multi-track passthrough (keeping director commentary, dubs, and original audio) is on the roadmap; today, only one audio track survives.
What happens to embedded subtitle tracks?
Text subtitles (SubRip, ASS, WebVTT, SSA) convert to mov_text so they play back inside the MP4. Bitmap subtitles (PGS from Blu-ray, VobSub from DVD) get dropped because MP4 has no container-level home for them - you would need to burn them into the video or ship them as a sidecar .srt file.
Are attached fonts or attachments in the MKV preserved?
No. MKV attachments (fonts used by styled subtitles, poster images, cover art files) have no MP4 equivalent and are stripped during the remux. This affects release-scene MKVs that ship fonts for typesetting subs; the MP4 will show subtitles in the player's default font instead.
Can I convert MKV to MP4 without VLC or HandBrake?
Yes - that is what this page is. VLC and HandBrake both do the job well, but they are 100 MB desktop installs; Wave runs the same remux in the browser tab you already have open. The underlying operation is identical: rewrite the container header, copy the H.264 or HEVC video stream and AAC audio stream through untouched (other audio formats, such as AC-3, get converted to AAC). No files leave your device either way; the only difference is the installer.
Is converting MKV to MP4 legal?
Yes for the files you own or have permission to convert. Container conversion is a technical operation on bytes you already have - it creates no copy of copyrighted content that did not already exist. Rip-tool origin (whether the source MKV came from a legally-acquired disc or a stream you had a right to record) is a separate legal question about the source, not about the container step.