Convert MKV to MP4 online
Convert an MKV file you already have on your device into an MP4 that plays on any player. If the video is already H.264 with AAC audio, it's an instant remux; if it carries other codecs (HEVC, AC3, DTS…), those tracks get re-encoded automatically to H.264/AAC. Nothing gets uploaded to any server.
Convert MKV to MP4
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Conversion details
How do you convert an MKV to MP4?
Choose the MKV file you want to convert to MP4.
Hit 'Convert'. If the video is already compatible (H.264/AAC), the remux takes seconds; if a track needs re-encoding, it takes a little longer.
Download the resulting MP4; it saves straight to your device.
If you already have a video saved in MKV format and need to play or share it on a device that doesn't handle it well, this converter turns it into MP4 right in your browser: if it's already H.264/AAC it repackages it instantly with no re-encoding, and if it carries other codecs it re-encodes only the track that needs it. We never upload the file to a server.
What's converting an MKV to MP4 good for?
MKV is the go-to container for movie and TV rips because it can hold multiple audio tracks, subtitles and high-quality codecs in a single file. The catch is that many phones, smart TVs, editing programs and social networks either play it poorly or reject it outright. MP4, on the other hand, plays pretty much everywhere. Converting to MP4 fixes that compatibility problem.
How it works: remux if it's compatible, re-encode if it's not
We use ffmpeg compiled to WebAssembly. First we check the MKV's codecs: if the video is already H.264 and the audio is AAC (or MP3), we do a remux (direct stream copy, '-c copy'), repackaging the same data inside an MP4 container without decompressing anything. That's nearly instant, even with files several GB in size, and there's no quality loss. If the video is HEVC/H.265 or the audio is AC3/DTS/TrueHD, we re-encode that specific track to H.264/AAC so the MP4 plays on any device: it takes longer and has a minimal quality loss, but it always works — it never fails.
Steps to convert your MKV to MP4
- Choose the MKV file from your device ('Choose file' button).
- Hit 'Convert'. The first time, your browser downloads the conversion engine (a few MB); after that, it's faster.
- Wait a few seconds for the remux to finish.
- Download the resulting MP4 with the 'Download result' button.
What carries over and what doesn't: subtitles and extra tracks
The main video and audio track carry over. What does not make it into the MP4 are subtitles embedded in ASS/SSA format (the typical fansub style, with custom colors and positioning) because MP4 doesn't support them, nor secondary audio tracks (only the main one). Audio in DTS, AC3 or TrueHD does convert: it gets re-encoded to AAC so it stays compatible, it isn't dropped.
Honest limits worth knowing
Since the process runs in your browser, the real limit is your device's memory, not an artificial cap of ours. With very large files (several GB), loading can take longer, especially on phones with limited RAM — try a computer if that happens. And if the MKV carries HEVC, AC3 or DTS, that track needs re-encoding: it doesn't fail, but it takes noticeably longer than a direct remux (the converter re-encodes on a single thread). For long HEVC videos, a computer handles it more comfortably.
Other related conversions
If your video comes in another container, you can also convert a MOV to MP4 (same system: remux if it's already H.264, re-encode otherwise) or a WebM to MP4 (this one always re-encodes). If all you want is the audio, try MP4 to MP3 once you have the file in MP4. If you don't have the video yet, use our YouTube video downloader.
Related articles
MP4 vs MKV: which format to choose when downloading a video
Choose MP4 for maximum compatibility (mobile, social, any player) and MKV only for 4K or videos with multiple audio/subtitle tracks. Here's the difference.
Convert a video to MP4: how to download it already in this format
Need a video in MP4? With PullVid you download it straight in MP4, the most compatible format. No conversion software: paste the link, pick MP4, and you're done.
MP4, WebM, MKV: Which Video Format to Choose When Downloading
Video format comparison: MP4, WebM, MKV, and AVI. Find out which one offers the best quality and compatibility for your downloads.
Frequently asked questions about MKV → MP4
Why is converting MKV to MP4 so fast?
When the MKV already carries H.264 video and AAC (or MP3) audio, the conversion is a remux: we don't re-encode anything, we just swap the container that wraps the data, so it takes seconds even with videos several GB in size. If the MKV uses other codecs (HEVC, AC3, DTS…), that track needs re-encoding and takes a bit longer.
Do I lose quality converting an MKV to MP4?
If the MKV is already H.264/AAC, nothing is lost: we copy the streams without re-encoding (remux), bit for bit identical to the original. If it carries HEVC, AC3 or DTS, we re-encode that track to H.264/AAC so the MP4 plays everywhere; that step involves a genuine re-encode with a minimal, practically unnoticeable quality loss.
Do subtitles and all audio tracks carry over?
The main video and audio track carry over. Subtitles embedded in ASS/SSA format (very common in fansub MKVs) don't make it into the MP4 because the container doesn't support them. Secondary audio tracks aren't included either; if the main one is DTS, AC3 or TrueHD, it gets re-encoded to AAC (not dropped) so it stays compatible.
Can the conversion fail because of the codec?
No. If the MKV carries video or audio that MP4 can't play as-is (HEVC, AC3, DTS, older codecs…), it doesn't fail: we automatically re-encode that track to H.264/AAC. The result is always a playable MP4; the only thing that changes is that re-encoding takes longer than a direct remux.
Is there a size limit on the MKV file?
We don't set an artificial limit, but the process runs in your browser: files above 2 GB can use a lot of memory just to load, especially on phones with limited RAM. The remux itself is fast; loading the file is usually the bottleneck, not converting it.
I don't have the MKV file yet — can I download it here?
This converter works with an MKV file you already have saved, from a download manager or a recording, say. Our YouTube video downloader delivers directly in MP4, so you usually won't need to convert anything after downloading with PullVid.