MP4 to MKV Converter

Convert MP4 to MKV (Step-by-Step)

This walkthrough is for anyone moving an MP4 into the Matroska (MKV) container — usually to build a Plex or Kodi library, bundle several audio languages or subtitle tracks into one file, or future-proof a clip with a modern codec. By the end you will have an MKV that plays the way you expect on your media server, and you will know which settings matter and which ones to leave alone.

How to Convert MP4 to MKV

1. Upload Your MP4 File: Drag and drop your file onto the page, or click "+ Add Files" to pick it from your computer. Both .mp4 and .m4v are accepted, and you can queue several files to convert in one batch with the same settings. Files are uploaded over an encrypted connection, processed on our servers, and deleted automatically after a few hours — no sign-up, never shared or made public. On a multi-gigabyte movie the bottleneck is upload time, not your computer's memory.
2. Pick Your Video and Audio Codec: Open Advanced Options to find the Video Codec and Audio Codec dropdowns. The default for MKV is H.264 video with AAC audio — the safest combination for direct playback. Because MKV is only a container, it does not lock you to a codec the way MP4 effectively does: choose H.265 (HEVC) for the smallest file if your player decodes it, AC3 or EAC3 for a home-theater receiver, or FLAC for lossless audio that MP4 cannot officially carry.
3. Set Quality, Resolution, and Trim (Optional): Leave the Quality Preset on "Very High (Recommended)" unless you have a reason to change it. To target an exact output size, switch File Compression to "Specific file size" rather than "Target file size (%)" — it is more predictable when you are also changing codec. Resolution defaults to "Keep original"; pick a preset only to downscale, and set Trim to "Time Range" to keep 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.

Common Errors and How to Fix Them

• Plex (or Kodi) transcodes the file instead of direct-playing it — Plex is largely container-agnostic; what triggers a transcode is usually the codecs inside, not the .mkv wrapper. If your TV cannot decode H.265 or your audio is DTS/TrueHD, the server re-encodes on the fly. Re-convert with H.264 video + AC3 audio to maximize direct play.
• iPhone or PlayStation Plex app refuses to play the MKV — some clients do not support the Matroska container for Direct Play. Plex will remux (copy the streams into a compatible container) when it can, but if it cannot, convert to MP4 instead with our MKV to MP4 converter.
• The browser won't open the MKV directly — that is expected. Chrome, Firefox, Edge, and Safari natively play MP4, WebM, and Ogg, not raw Matroska, so use VLC, Kodi, or Plex to view the file.
• The MKV is too large to share — Matroska adds almost no overhead, so a big MKV means a high source bitrate. Shrink it with our MKV compressor before sending.
• Subtitles or extra audio tracks went missing — the converter writes the streams it reads from the source; soft subtitles burned only into a separate sidecar .srt are not pulled in automatically. Mux them in your media tool, or keep the sidecar file next to the MKV.

When This Doesn't Work

A straight container change cannot rescue a file the source already broke. DRM-protected purchases (iTunes .m4v rentals, for example) will not convert because the stream is encrypted, and a truncated or partially downloaded MP4 may convert but still stutter at the damaged point. If you only need the audio, it is cleaner to extract that track separately than to wrap a damaged video. And if a specific device still refuses the result, the pragmatic move is to convert to MP4 with H.264/AAC — the most universally supported combination — rather than chase MKV compatibility on hardware that was never built for it.

Frequently Asked Questions

Does converting MP4 to MKV reduce quality?

It depends on whether the streams are copied or re-encoded. MKV is a container, so the ideal is a rewrap that keeps the original H.264 video bit-for-bit. By default this converter re-encodes to H.264 video and AAC audio at a high quality preset, so any loss is small and visually hard to spot; if you need a guaranteed-lossless copy, keep the codec the same as the source and use the highest quality setting.

What can MKV do that MP4 can't?

The Matroska container can hold many video, audio, and subtitle streams in one file and supports codecs MP4 does not officially carry, such as FLAC audio and a wider range of subtitle formats. That flexibility is why MKV is popular for movie libraries with multiple language tracks. MP4's advantage is the reverse: near-universal device and browser support.

Is MKV better than MP4 for Plex?

Neither is strictly better — Plex does not really care about the container, only the codecs inside it. MP4 with H.264 and AAC direct-plays on the widest set of clients, including iPhones and game consoles that may not accept MKV. MKV is the better choice when you specifically need multiple audio tracks or subtitles bundled in one file and your playback devices handle it.

In our testing, how much does the file size change going from MP4 to MKV?

In our testing, because this tool re-encodes to H.264/AAC at the "Very High" preset rather than ballooning the data, the MKV lands close to the source MP4's size — the Matroska container itself adds only a few kilobytes of overhead. If your MKV comes out dramatically larger, the cause is a higher-bitrate or lossless codec choice, not the container.

Can a browser play the MKV file I download?

Not directly. Per MDN's container documentation, browsers natively support 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 instead, or convert it to MP4 if you need in-browser playback.

Will my AC3 or DTS surround audio survive the conversion?

MKV can carry AC3, EAC3, and DTS tracks, so the container itself preserves surround audio. Whether it plays back as surround 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 and may be downmixed or transcoded on playback.