Is WebM to MKV conversion lossless?

Yes. Since WebM is a subset of Matroska, the conversion is a simple remux that takes the existing VP8/VP9 video and Vorbis/Opus audio streams and places them in the MKV container with no re-encoding.

Why convert WebM to MKV if they are related formats?

MKV supports features that WebM does not, including a broader codec range, more subtitle formats, and better compatibility with desktop media players and media servers.

Will Plex or Kodi handle the MKV file better than WebM?

Generally yes. Media servers like Plex and Kodi have deeper MKV support and may handle metadata, chapters, and track selection better with MKV than with WebM.

Can I add additional audio or subtitle tracks to the MKV?

Yes. After conversion, you can mux additional tracks into the MKV file using MKVToolNix or similar tools, which is not possible with the restricted WebM container.

How fast is WebM to MKV conversion?

Nearly instant. Since both formats share the Matroska base, no transcoding is required. The file is simply repackaged, which takes seconds regardless of file size.

WebM to MKV Converter

WebM is technically a restricted subset of the Matroska format, so converting to MKV unlocks the full Matroska feature set. As an MKV file, your video gains support for additional audio tracks, subtitle streams, chapter markers, and a broader range of codecs, all while playing in media players that support MKV but may not recognize WebM.

100% Private — Your files never leave your device

Convert MKV to WebM instead

Since WebM is a restricted profile of the Matroska format, this conversion is a perfect lossless remux. The VP8/VP9 video and Vorbis/Opus audio streams are transferred directly into the MKV container with zero quality change and near-instant processing.

How to Convert WebM to MKV

1
Upload your WebM video file to the converter.
2
Select MKV as your output format.
3
Click Convert and download your full-featured MKV file.

WebM vs MKV — Format Comparison

Feature	WebM	MKV
Full Name	WebM Video Format	Matroska Video
Type	Video	Video
Codec	VP8 / VP9 / AV1	H.264 / H.265 / VP9 / AV1
Lossless	No	No
Extension	.webm	.mkv
Best For	HTML5 web video	High-quality video archival

WebM Strengths

Royalty-free and open source
Excellent compression for web streaming
Native support in all modern browsers

MKV Strengths

Open standard with no licensing fees
Supports unlimited tracks (audio, subtitles, video)
Excellent for high-quality video with multiple languages

Frequently Asked Questions

When to Convert WebM to MKV

Adding the full MKV feature set to web-downloaded VP9 videos
Preparing WebM recordings for inclusion in a Plex or Kodi media library
Enabling additional subtitle and audio track support for WebM content
Consolidating WebM files into a standardized MKV media collection

Related Conversions

WebMMP4 WebMMOV WebMAVI WebMMP3 MP4MKV MOVMKV AVIMKV

About Our WebM to MKV Converter

This converter uses FFmpeg compiled to WebAssembly (WASM) to transform your WebM files into MKV format entirely inside your web browser. When you select a file for conversion, the processing happens locally on your device. Your file is never uploaded to any server, never transmitted over the internet, and never stored anywhere outside your own computer. This privacy-first architecture makes our tool safe for confidential and sensitive content — including legal recordings, medical files, financial presentations, personal media, and any other material you would not want on a third-party server.

The conversion process works by decoding the WebM input using FFmpeg's highly optimized decoders, then encoding the output in MKV format with carefully tuned settings that balance quality and file size. When possible, the tool uses stream copy mode to remux (rewrap) compatible streams without re-encoding, which preserves the original quality with zero generational loss. When re-encoding is necessary due to codec differences between the two formats, the converter uses high-quality encoding settings to minimize any visible or audible quality reduction.

Supported input formats include MP4, MOV, AVI, MKV, and WebM for video, with automatic detection of the codecs and parameters inside your source file. The output is optimized for maximum compatibility with modern devices and platforms including Windows, macOS, Linux, iOS, Android, smart TVs, and all major web browsers.

Why browser-based conversion is safer: traditional online converters require you to upload your file to a remote server, where it is processed by third-party infrastructure and then made available for download. This workflow exposes your data to interception during upload, unauthorized access on the server, unclear data retention policies, and potential data breaches. Professionals in regulated industries such as healthcare (HIPAA), legal (attorney-client privilege), and finance (SOX, GDPR) often cannot use server-based converters without violating compliance requirements. Our WebAssembly-powered approach eliminates all of these risks by ensuring your data never leaves the browser sandbox on your own device.

The tool handles files up to 500 MB on desktop browsers and 200 MB on mobile devices. No account registration, no software installation, and no recurring subscription is required. Conversion typically completes in seconds for small files and a few minutes for larger ones, depending on your device's processing capabilities. Once the conversion is finished, you can download the result immediately and the source data is discarded from browser memory.