Does the picture quality drop if I turn my MP4 into an MKV?

Almost never. Because the MKV container can carry every video codec you will meet in practice, such as H.264, H.265 and AV1, there is no need to re-encode the picture data inside the MP4. What happens during the conversion is that the audio and video tracks are taken out of the old container and copied into the new one exactly as they are; this is called a remux. Because bit-for-bit identical data is carried across, the quality loss is zero.

How long does the conversion take?

A remux is very fast compared with re-encoding, because the compression algorithm never runs and only data is copied. What really sets the time is the size of the file and the read-write speed of the disk. Even so, on a long film file of several gigabytes, the upload, processing and download steps can add up to minutes, so it is worth being patient with large files.

My television will not open my MKV file, what should I do?

This is MKV's best-known weak point. Many smart televisions, game consoles and above all the default players on iOS devices do not recognise the MKV container, or recognise it only with certain codecs. If you are going to play the file on a device of that kind, staying with MP4 is the safer preference. Alternatively you can use a container-agnostic player such as VLC on your computer, or watch through a media server that streams to the device.

My MP4 had more than one audio track — did they all come across to the MKV?

In a properly performed remux, every existing track is carried into the new container. MKV is far more relaxed about this than MP4; it carries multiple audio tracks and subtitles without trouble. Still, we recommend opening the file after conversion with a tool such as VLC or MediaInfo and checking the track list. If a track is missing, it is likely that the track in question was stored in a non-standard way inside the source MP4.

A Guide to Converting an MP4 File to MKV: Step by Step and Lossless

Why this conversion is different from the others

Video conversion generally brings to mind long waits and some quality loss. The move from MP4 to MKV is the clearest exception to that rule, and understanding the reason is also the key to doing it properly.

MP4 and MKV are both containers. In other words, the file extension tells you which box the picture is stored in, not how it was compressed. The actual data inside has been compressed with a codec: in MP4 files that is usually H.264 or H.265 video and AAC audio.

Here is the crux of it: the MKV container was designed to be able to carry practically any codec. So to put the H.264 picture inside an MP4 into an MKV, you do not have to break it apart and recompress it. The data is copied into the new container as it is. This operation is called a remux and it has two consequences: there is no quality loss, and the operation finishes quickly.

Before you start converting: know what you will get

Take a minute and clarify this question: why are you moving to MKV?

If you want to add more than one audio track or subtitle you are in the right place, this is MKV's greatest strength.
If you want to archive it is sensible too; MKV comfortably carries chapter markers and rich metadata.
If you are moving simply because you heard MKV is higher quality then stop. The container does not change quality. The same picture stays at the same quality, only the box changes.
If you are going to play the file on a television or a phone think about your device's MKV support before doing the conversion; we have given that topic a separate section below.

Step-by-step conversion

1. Prepare the source file

Make sure the MP4 you are going to convert is complete and not corrupt. A file that was half downloaded or cut off during recording will throw an error during conversion or produce incomplete output. Open the file in a player and fast-forward to the end; if the end plays properly it is probably sound.

2. Upload the file

Drag and drop your file into our MP4 to MKV converter. With large files the upload step can take longer than the operation itself; this is entirely to do with the upload speed of your internet connection.

3. Start the conversion and wait

Because no compression is done during a remux, the operation takes far less time than re-encoding the same file would. Even so, allow for the whole process taking minutes on files the size of a full-length film. Wait without closing the tab.

4. Download the output

The downloaded file will have the extension .mkv and its size will come out very close to the source MP4. The size being almost identical is a good sign — it shows that the data was carried across without being recompressed.

Three checks to run after conversion

You have downloaded the file and the job looks finished. But before filing it away in your archive, quickly verify these three things:

Duration check. Open the new MKV file in a player and see that its total duration is the same as the source MP4's. If there is a shortfall in the duration, the file has been truncated.

Audio sync check. Watch a few seconds from the beginning, the middle and the end of the video. Make sure in particular that the audio and picture have not drifted apart towards the end. Sync problems are rare in remux operations, but they can arise if the source file has a variable frame rate (some screen recordings do).

Track list check. Open the file from Tools > Media Information in VLC, or with a program such as MediaInfo, and look at how many audio and subtitle tracks there are. If the source has two audio tracks you should see two in the output as well.

These three checks take a minute of your time and protect you from the surprise, months later, of finding that the file in your archive was broken.

Why did the size not go down?

This is one of the most frequently asked questions. The user looks at the file size after the conversion, sees that it has hardly changed at all and thinks something has gone wrong.

In fact everything went right. Changing the container does no compression, and therefore does not shrink the file. The tiny differences in size come only from the containers' own header structures taking up slightly different amounts of space. If you genuinely want to shrink the file, what you need to do is not change the container but recompress the video with a more efficient codec — and that operation brings quality loss.

Problems that come up often

The file will not open in some programs. This is MKV's known limit. Even though the container itself is technically superior, the breadth of support lags behind MP4. The built-in players in smart televisions, the default video app on the iPhone and iPad, and some video editing programs struggle with MKV. Using VLC or MPV on a computer solves the bulk of these problems.

Subtitles do not appear. Subtitles inside an MKV are stored as separate tracks and the player may show them switched off by default. You may need to select the relevant track by hand from the player's subtitle menu. Burning the subtitle permanently into the picture is an entirely different operation and requires re-encoding.

The operation keeps failing with an error. Usually the source file is corrupt. Try playing the same MP4 in a player from beginning to end; if it stalls at a particular point, the problem is in the file rather than in the converter.

There are accented characters in the file name. Rarely, special characters can cause trouble in some processing chains. If you are having problems, simplify the file name and try again.

Enriching the file after conversion

The real gain from moving to MKV emerges after the conversion is finished. You now have a container that can carry tracks flexibly, and you can add to it later.

The most common scenario is adding subtitles. If there is a .srt subtitle sitting next to the file in your folder, you can place it inside the MKV as a separate track. The subtitle then travels with the file; when you move the video to another disk there is no chance of forgetting the subtitle. This addition never touches the picture, and therefore creates no new quality loss.

The same logic applies to a second audio track. If you have the original language and the dubbed version of a film sitting in separate files, you can gather the audio tracks under a single MKV. Do not forget to add a language tag to the tracks; players look at those tags and select the right track by themselves, so you do not have to fight with a menu every time you open it.

The only point to watch here is this: adding a subtitle as a track and burning it into the picture are different operations. A subtitle added as a track can be switched on and off and requires no re-encoding. Writing it permanently into the picture requires re-encoding the video from beginning to end; that takes a long time and brings some quality loss. The advantage MKV offers is the first one, so stay on that road as far as you can.

In summary

The move from MP4 to MKV is one of the most harmless operations in the video world: the picture data is not touched, the quality is preserved exactly, and the operation finishes quickly compared with re-encoding. What you gain is a flexible container that can hold many audio and subtitle tracks in a single file — ideal for archiving. The price you pay in return is compatibility: not every device opens MKV. If you are converting in order to store the file, MKV is an excellent choice; if you are converting in order to send it to someone else or play it on a television, it is worth thinking twice.