Convert WEBM to MKV online

To convert WEBM to MKV, upload the WEBM file, start the conversion and download the result: Converter.now writes an MKV with H.264 video (CRF 23) and Opus audio at 160 kbps. The first video and audio tracks are re-encoded; extra audio tracks and embedded subtitles are not copied. Both formats are lossy, so this adds a second round of compression; convert from the best original you have rather than an earlier export.

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WEBM to MKV at a glance

Converts: WebM video → Matroska video
Output: An MKV with H.264 video (CRF 23) and Opus audio at 160 kbps
Compression: WEBM: lossy → MKV: lossy
Engine: FFmpeg
Privacy: Private download link; files are deleted after 6 hours. No account needed.

How WEBM turns into MKV: Timed-stream transcoding

Decoding WEBM recovers the picture, sound, and timing streams it carries; writing MKV means re-expressing that under a different codec and container profile. Duration commonly survives untouched even while compression, channel mapping, cadence, metadata, or auxiliary streams do not. WebM parsing follows Matroska-derived clusters and cues while enforcing web-oriented VP8/VP9/AV1 picture and Vorbis/Opus audio expectations. Matroska delivery can retain multiple timed tracks, languages, subtitles, chapters, and attachments, though simplified writers and browser players may use only a subset.

Decoded media - An open web container written here with VP9 video and Opus audio when available. Encoded delivery profile - A flexible Matroska container capable of multiple audio, video, subtitle, and attachment streams.

For related jobs, compare other WEBM converters with the full list of MKV conversions.

Before converting a whole batch, read the practical guide to web video codecs; it explains how to balance reach, quality and file weight.

WEBM

WebM video

Used for open web video delivery.

Family: Video
MIME: video/webm

→

MKV

Matroska video

Designed for flexible multi-track video storage.

Family: Video
MIME: video/x-matroska

Reading WEBM correctly and satisfying what MKV expects

What can fail when WebM video is interpreted

Encoding can be slower and older Apple or editing software may not support WebM reliably.

What Matroska video cannot safely promise

A simplified conversion may keep only primary streams, and many browsers do not play MKV directly.

How the MKV writer handles this route

The verified MKV profile uses H.264 video and Opus audio when those streams are present.

Acceptance evidence for the converted MKV

Record the WEBM evidence before conversion

Test browser playback, seeking, VP9/Opus support, dimensions, and audio sync.

Test the MKV file in its receiving workflow

Inspect the stream list, subtitles, language labels, chapters, seeking, and sync.

Deciding when WEBM to MKV is the right conversion

Choose MKV only after the player, editor, or publishing service confirms its container and codecs; keep WEBM if extra tracks or source encoding must survive.

What changes between the WEBM model and the MKV deliverable

The first video and audio tracks are re-encoded; extra audio tracks and embedded subtitles are not copied.
Both formats are lossy, so this adds a second round of compression; convert from the best original you have rather than an earlier export.
Treat WEBM-specific metadata fields as optional in the MKV result until you have actually confirmed they survived.