Does converting WebM to MKV lose quality?

No — if you choose the remux mode, quality is identical to the original. The streams are copied without any re-encoding. Quality loss only occurs if you explicitly choose to re-encode the video with a different codec such as H.264 or H.265.

Do I need to install anything?

Yes. This tool generates FFmpeg commands. FFmpeg is a free, open-source tool available for Windows, macOS, and Linux. Download it from ffmpeg.org and add it to your system PATH.

How long does conversion take?

A remux is nearly instant — often faster than real-time because it copies data without decoding or encoding. Re-encoding with H.264 or H.265 takes longer: typically 0.5x to 5x the video duration depending on your CPU speed and quality settings.

Why convert WebM to MKV if they are both Matroska-based?

WebM is a strict subset of Matroska with limited codec support (VP8/VP9/AV1, Opus/Vorbis only) and no support for multiple audio tracks, subtitle tracks, or chapters. MKV lifts all those restrictions, making it far more flexible for media libraries.

Will my subtitles and audio tracks be preserved?

Yes. Subtitle and audio tracks present in the WebM file are preserved during remux. You can also attach external subtitle files using the -i flag in FFmpeg before the output filename.

Can I batch convert multiple WebM files?

Yes. On Linux/macOS: for f in *.webm; do ffmpeg -i "$f" -c copy "${f%.webm}.mkv"; done. On Windows CMD: for %f in (*.webm) do ffmpeg -i "%f" -c copy "%~nf.mkv"

WebM to MKV Converter

Name: WebM to MKV Converter
Availability: InStock
Author: Nham Vu

Generate a ready-to-run FFmpeg command to convert WebM video files to MKV format — remux or re-encode, with audio and quality options.

FFmpeg Command Builder

Fill in the fields and click Build Command to get a ready-to-run FFmpeg command.

Input filename

Output filename

Conversion mode

Audio handling

Generated Command

ffmpeg -i video.webm -c copy video.mkv

Remuxes the WebM container into MKV without re-encoding. All streams are copied as-is — lossless and very fast.

WebM vs MKV — Format Comparison

Feature	WebM	MKV
Released	2010 (Google)	2002 (Open source)
Based on	Matroska subset	Matroska (full)
Video codecs	VP8, VP9, AV1 only	All codecs (H.264, H.265, AV1, VP9...)
Audio codecs	Opus, Vorbis only	All codecs (AAC, AC3, FLAC, MP3...)
Multiple audio tracks	Limited	Unlimited
Subtitle tracks	WebVTT only	Unlimited (SRT, ASS, PGS...)
Chapter support	Limited	Full
HDR / metadata	Partial	Full

Quick FFmpeg Reference

Remux (lossless, fastest)

ffmpeg -i input.webm -c copy output.mkv

Re-encode to H.264 + AAC

ffmpeg -i input.webm -c:v libx264 -crf 23 -c:a aac output.mkv

Re-encode to H.265 + AAC

ffmpeg -i input.webm -c:v libx265 -crf 28 -c:a aac output.mkv

Add external subtitle file

ffmpeg -i input.webm -i subs.srt -c copy -c:s srt output.mkv

Batch convert (Linux/macOS)

for f in *.webm; do ffmpeg -i "$f" -c copy "${f%.webm}.mkv"; done

Batch convert (Windows CMD)

for %f in (*.webm) do ffmpeg -i "%f" -c copy "%~nf.mkv"

Copied!

Summary

Generate a ready-to-run FFmpeg command to convert WebM video files to MKV format — remux or re-encode, with audio and quality options.

How it works

WebM files store VP8/VP9/AV1 video and Opus/Vorbis audio in a restricted subset of the Matroska container.
MKV uses the same underlying Matroska format but with no codec restrictions and full metadata support.
Because both formats share Matroska roots, FFmpeg can remux WebM to MKV in seconds by simply relabeling the container — no re-encoding needed.
Choose a conversion mode in the builder below (remux or re-encode), set your filenames, then click Build Command.
Copy the generated command and run it in your terminal where FFmpeg is installed.
For batch conversion, use the shell loop examples in the Quick Reference section.

Use cases

Remux WebM files to MKV to unlock multi-track audio and subtitle support.
Preserve VP9 or AV1 video quality while gaining the flexibility of the MKV container.
Add external subtitle files (.srt, .ass) to a WebM video by converting it to MKV.
Improve compatibility with media players and NAS devices that handle MKV better than WebM.
Archive WebM recordings in the future-proof, widely supported MKV format.
Re-encode WebM to H.264 inside MKV for maximum player compatibility on older devices.
Batch-convert a folder of WebM downloads to MKV with a single FFmpeg loop.
Enable chapter markers and cover art not supported in the WebM format.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu