Tech

MKV vs MP4: Differences, Which to Use, and How to Convert MKV to MP4

MKV and MP4 can hold the same video in the same quality. What really differs, when MP4 is the better choice, and how to convert MKV to MP4 without losing quality.

November 12, 2026 · 4 min read · RunConvert Team

You download a film or record your screen, and the file ends in .mkv. It plays fine in VLC on your laptop. Then you try it on your iPhone, your TV, in a video editor or on a website, and it won't open, or it plays without sound. Converting it to MP4 almost always fixes that.

This article explains what actually differs between MKV and MP4, which one to use, and how to convert MKV to MP4 without losing quality. The short version of the steps is in our MKV to MP4 guide.

Both Are Containers

MKV (Matroska Video) and MP4 (MPEG-4 Part 14) are both containers: boxes that hold a video stream, one or more audio streams, subtitles and information about the file. Neither one decides how good the video looks. That's down to the codec used inside the box, such as H.264, H.265 (HEVC), VP9 or AV1.

That's why an MKV and an MP4 can look exactly the same. If both hold the same H.264 video, the picture is identical; only the box differs.

The Real Differences

	MKV	MP4
Plays on iPhone, iPad and Safari	No	Yes
Plays on Android, Windows and Mac	Often needs a player like VLC	Yes, built in
Smart TVs and game consoles	Some	Almost all
Websites and social media uploads	Rarely accepted	Accepted everywhere
Video editors	Limited	Yes
Several audio languages	Yes	Yes
Subtitle tracks	Yes, many kinds	Yes, fewer kinds
Audio codecs	Almost any (DTS, FLAC, AC3, AAC…)	Mainly AAC, AC3, MP3
Open standard	Yes	Yes

In short: MKV is more flexible, MP4 is more compatible. MKV can hold almost any codec and many subtitle formats, which is why it's popular for films and archives. MP4 is supported by practically every device, app and website.

Which Should You Use?

Keep MKV for a personal film collection you watch in VLC, Plex or Kodi, especially with several languages or subtitle tracks.
Use MP4 for anything you share, upload, edit or watch on a phone or TV. If you're not sure, MP4 is the safe choice.

Why MKV Files Won't Play (or Have No Sound)

Two different things can go wrong:

The container isn't supported. iPhones, Safari, many TVs and most editors simply don't open .mkv files.
The audio isn't supported. Films in MKV often use DTS, AC3 or FLAC sound. The video plays, but phones and browsers can't decode the audio, so it's silent.

Converting to MP4 with AAC audio fixes both.

How to Convert MKV to MP4 Without Losing Quality

Because most MKV files already contain H.264 or H.265 video, which MP4 supports, the video can be moved into an MP4 box without re-encoding. That's called remuxing: it's fast, and the picture stays exactly the same.

Open the MKV to MP4 converter and add your files (up to 20 at a time, 1 GB in total).
Click the settings button next to the format and set Video codec to copy.
Leave the audio at its default, so DTS or FLAC sound becomes AAC that plays everywhere.
Click Convert and download your MP4s.

If the MKV uses a codec MP4 players handle poorly, such as VP9 or AV1, skip step 2. With the default settings the video is re-encoded to H.264 at high quality, and the difference is hard to see.

Making Big MKV Files Smaller

MKV films are often several gigabytes. In the same settings you can choose a smaller resolution (1080p or 720p) or a lower quality value to cut the size, and trim the start and end to keep only the part you need.

Don't Rename .mkv to .mp4

Renaming the file changes only its name, not what's inside. Most players check the contents, find an MKV and refuse to play it. Always convert instead.

Converting a Whole Folder

For a few files, the online converter is quickest. For a large library, the RunConvert CLI converts every video in a folder from the terminal:

runconvert convert ~/Movies --only video --to mp4

Summary

MKV and MP4 can hold the same video in the same quality. MKV is the flexible format for collections; MP4 is the one that plays everywhere. When an MKV won't open or has no sound, convert it to MP4, and set Video codec to copy to keep the original quality. Working with iPhone videos instead? See MOV vs MP4.