Free Online MP4 to MKV Converter
Convert MP4 to MKV locally. Save a Matroska copy with H.264 picture and a readable soundtrack, and choose whether to keep the original frame size.
Drop your MP4 video here
or choose a local file or import from a URL
Choose a file to get started
How to convert MP4 to MKV
Select the source MP4
Open one .mp4 file. If it has alternate languages or commentary, check the source track list before conversion. The settings panel lets you choose a resolution and does not offer a manual audio or subtitle selector.
Set the maximum height
Choose Original for the existing dimensions, or set a lower height to reduce the frame. For a 1920 × 1080 landscape recording with square pixels, a 720p ceiling produces 1280 × 720. A smaller source stays below the ceiling.
Download the Matroska file
Start the MP4 to MKV conversion and save the result. Use a player with Matroska support to open the downloaded file, select the available audio and seek to a point beyond the beginning of the video.
MP4 to MKV stream comparison at original resolution
This MP4 to MKV example starts with one H.264 video track and one AAC soundtrack. The two files can show identical codec names and frame dimensions in a player, even though one uses an MP4 container and the other uses Matroska.
MP4
- 1920 × 1080
- H.264 + AAC
MKV
- 1920 × 1080
- H.264 + AAC
Which video, audio and subtitle tracks does the MKV contain?
Video stream
The MKV output targets H.264. If the input already contains compatible video and no resize is needed, the native conversion path may reuse it. Incompatible streams, scaling or the fallback encoder can require new frames. Leaving resolution on Original preserves the intended frame dimensions, without promising that every encoded video packet remains identical.
Audio stream
The output targets AAC when the source has readable sound. A stereo track contains two audio channels within a single stream. A second language or a commentary mix can be a separate stream entirely. The converter is designed for a main picture and soundtrack. If retaining every language is a delivery requirement, keep the multi-track original and verify the output stream list before using this copy.
Subtitles and on-screen text
Converting MP4 to MKV keeps titles already drawn into the picture. A selectable caption track can be disabled in a player and is separate from those pixels. An external SRT loaded automatically from the source folder is a third case. This page takes one video input. It provides no control for importing a second subtitle file into the new Matroska container.
How to check the saved MKV before adding it to a library
Identify the soundtrack
If several source tracks start with the same opening music, seek to dialogue before comparing them. Check the language and the audio options the destination player actually offers for the saved MKV.
Output size
Repackaging compatible compressed streams can give a file close in size to the original because most of the bytes belong to video and audio. Re-encoding can change the size in either direction. The format choice alone gives no target compression ratio. Compare the actual sizes displayed in the result panel and use the compressor separately if an upload limit is the reason for changing the file.
Check the library entry
Open the downloaded file directly first. Then play it through the library app. If the two differ, confirm the library indexed the new file and inspect the client’s playback status before converting the source again.
Why does an MKV play on the desktop but fail on the TV?
Check the TV’s codecs
Start with the destination player’s requirements. Container support tells you whether it recognizes Matroska, while video and audio codec support determine which streams it can decode. The MP4 to MKV preset produces a specific combination. If the playback requirement calls for a different audio format or for a different video codec, changing only the container name is insufficient.
Read the media server’s playback status
For server playback, open the client’s playback information while the MKV is playing, if that information is available. Direct playback, repackaging on the server and re-encoding are different operations. Check which stream is being processed and which audio and subtitle options are active. If only the sound is being converted, reducing picture height changes a setting unrelated to the audio requirement.
Compare the active language and subtitle tracks
For a missing language, first compare the source audio track list with the output track list. A player may remember a preferred language on the source but select a different available track on the download. Listen to the same spoken passage in both files. For subtitles, move a copy of the video into a folder without its external caption file and open it directly to identify what the MKV itself contains.
Find the point where playback stops
When playback stops at a consistent time, check that location in the original MP4. Play from just before the interruption and then try seeking near the end. A missing end segment cannot be reconstructed by choosing a new container. If the downloaded MKV plays through in a compatible desktop player, investigate the device or server playback path before changing the export settings again.
MP4 to MKV playback and subtitle questions
Will my TV Direct Play the MKV?
Test it with the TV app that will actually be used. Start with the main audio, original playback quality and subtitles off, then enable your usual settings one at a time. The same server can deliver different playback modes to two clients.
Does the MKV keep both a commentary track and the film mix?
The MP4 to MKV preset targets a main soundtrack. To deliver both selectable tracks, inspect the input and use a multiplexer with explicit track selection. Play the same scene twice in the saved file, changing the audio selection between plays.
Can I switch back to MP4 for a player that rejects Matroska?
Use the original MP4 if you still have it. When the MKV is your only copy, the MKV to MP4 converter can create a viewing file. Reversing an MP4 to MKV conversion does not restore a language track omitted from the first export.
Can I convert MP4 to MKV and add an SRT at the same time?
The converter accepts the video as its input. For a selectable subtitle inside the MKV, use a multiplexer and add the SRT as a separate track. Verify its language, timing and selectable status in the finished file.
Why does my downloaded MKV open in a player but not in the browser preview?
An MP4 to MKV conversion can finish successfully even if the browser cannot preview the resulting container. Try opening the saved MKV directly in a Matroska-capable player. If it also fails there, check that the source plays through the same passage before exporting again.