MP4 to MKV Converter

MP4 to MKV — Which Container Should You Convert To?

You are moving an MP4 into the Matroska (MKV) container, usually to bundle multiple audio languages and toggleable subtitles into one file, to use a codec MP4 does not officially carry, or to build a Plex or Kodi library on an open, future-proof format. One thing to know up front: this converter re-encodes the video rather than stream-copying it, so your MKV is a fresh H.264 encode at a high-quality preset — not a bit-for-bit copy of the original MP4 stream. If you already know you want MKV, jump to the steps; if you are still deciding, the comparison below settles it.

MP4 vs MKV, Side by Side

When to Convert MP4 to MKV

• You want several audio languages or a commentary track selectable in one file.
• You need soft, toggleable subtitles instead of subtitles burned into the picture.
• You are archiving to an open, royalty-free container that is not tied to a single vendor.
• Your target codec or audio format (FLAC lossless audio, for example) is awkward to carry inside MP4.
• You are feeding a Plex, Jellyfin, or Kodi library where the extra track flexibility matters.

When to Stay on MP4

• Your video has to play on iPhones, smart TVs, game consoles, or car head units without a hitch.
• You are uploading to the web or embedding in a page — browsers play MP4 natively but not raw MKV.
• You want the single most broadly compatible file and only need one audio track.
• You are sharing by phone or messaging app, where MP4 is expected and MKV often is not previewed.

How to Convert MP4 to MKV

1. Upload Your MP4 File: Drag and drop onto the page or click "Add Files" to pick from your computer. Both .mp4 and .m4v are accepted, and you can queue several files to convert as a batch. Files upload over an encrypted connection, are processed on our servers, and are deleted automatically after a few hours — no sign-up, never shared or made public. On a multi-gigabyte movie the wait is upload time, not your device.
2. Choose Video and Audio Codecs: Open Advanced Options and set the Video Codec and Audio Codec dropdowns. MKV defaults to H.264 video with AAC audio — the safest pairing for direct play. Because MKV is only a container, you are free to pick H.265 for a smaller file, AC3 or EAC3 for a home-theater receiver, or FLAC for lossless audio.
3. Set Quality, Resolution, or Trim (Optional): Leave File Compression on the "Quality Preset" of "Very High (Recommended)" for near-source quality, or switch it to "Specific file size" when you need a predictable output size. Video resolution stays on "Keep original" unless you pick a preset to downscale, and Trim's "Time Range" keeps just a clip.
4. Convert and Download: Click "Convert" and download your .mkv when it finishes. No sign-up, no watermark, no quality cap on the free conversion.

Choosing Codecs for Your MKV

Because the conversion re-encodes rather than copies, the codecs you pick decide both quality and file size. Use this as a quick guide inside the Video Codec and Audio Codec dropdowns.

Frequently Asked Questions

Does converting MP4 to MKV reduce quality?

This converter re-encodes the video, so it is not a bit-for-bit copy of your source. In our testing, the default "Very High" preset re-encodes with H.264 at a visually lossless CRF tier: the picture is hard to tell apart from the source at normal viewing distance, but because it is a fresh encode rather than a stream copy, expect the output size and bytes to differ somewhat from the original rather than match it exactly. Keep the preset high and use the same video codec as your source to keep loss negligible.

What can MKV do that MP4 can't?

Matroska can hold many video, audio, and subtitle streams in one file, supports toggleable soft subtitles (SRT, ASS, PGS), carries chapters, and accepts codecs MP4 does not officially support, such as FLAC lossless audio. That flexibility is why it is popular for multi-language movie libraries. MP4's counter-advantage is near-universal device and browser support.

Is MKV better than MP4 for Plex?

Plex is largely container-agnostic — what decides direct play versus a transcode is the codecs inside, not the .mkv wrapper. MP4 with H.264 and AAC direct-plays on the widest set of clients, including iPhones and consoles that may reject MKV. MKV wins when you specifically need bundled multi-audio or subtitle tracks and your playback devices handle the container.

Can a browser play the MKV file I download?

Not directly. Per MDN's media container documentation, browsers natively play MP4, WebM, and Ogg in the video element, and WebM is a web-optimized subset of Matroska — full MKV is not on that list. Open the file in VLC, Kodi, or Plex, or convert it back with our MKV to MP4 converter if you need in-browser playback.

Will my AC3 or DTS surround audio survive the conversion?

MKV can carry AC3, EAC3, and DTS, but remember the audio is re-encoded to whatever you set in the Audio Codec dropdown — the default is AAC. To keep a surround track, select AC3 or EAC3 explicitly. Whether it plays back as surround then depends on the destination: AC3 and EAC3 pass through most AV receivers, while DTS and TrueHD are decoded less reliably by smart TVs and mobile apps.

Does converting to MKV make the file bigger?

Not because of the container — Matroska's own overhead is negligible. Output size is driven by the codec and preset you choose during the re-encode. Keep the video codec the same as your source at the "Very High" preset and the result lands close to the original; switch to a lossless or higher-bitrate codec and it will grow. If your MKV is too large to share, shrink it with our MKV compressor.

Will my subtitles and multiple audio tracks carry over automatically?

The converter writes the streams it reads from your source file, so audio and embedded (soft) subtitle tracks already inside the MP4 are carried into the MKV. A subtitle that lives only in a separate sidecar .srt next to the video is not pulled in automatically — mux it in your media tool or keep the sidecar file alongside the MKV.

Is MKV an open format, and is it still maintained?

Yes. Matroska has been developed as an open, royalty-free standard by the Matroska project since 2002, and the container was formally standardized by the IETF as RFC 9559 in 2024. That open, actively maintained status is a large part of why archivists and media-server users favor it over proprietary containers.