What is WebM and why convert MKV to it?

WebM is an open-source, royalty-free video container developed by Google, based on the Matroska format. It supports VP8, VP9, and AV1 video with Vorbis or Opus audio. All major browsers (Chrome, Firefox, Edge, Safari 14.1+) play WebM natively. MKV itself is not supported by most browsers or iOS devices. Converting MKV to WebM lets you embed video directly in HTML5 without a plugin or external player.

Which codec should I choose — VP8, VP9, or AV1?

VP9 is the best choice for most users: it compresses 40-50% better than VP8 and encodes in reasonable time. VP8 is faster to encode and has the widest compatibility including older browsers. AV1 offers the best compression (roughly 20-30% better than VP9) but encoding is much slower — good for archiving or pre-rendered content. For web video, VP9 with Opus audio is the recommended combination.

Will converting MKV to WebM reduce quality?

Re-encoding always introduces some generation loss, but at low CRF values (15-25 for VP9) the difference is imperceptible. If your MKV already contains VP8 or VP9 video, you can use -c:v copy to avoid re-encoding the video stream entirely. For audio, if the MKV already has Opus or Vorbis audio, select "Stream copy audio" to skip re-encoding that track too.

Why does my WebM file have no audio?

WebM supports only Vorbis and Opus audio codecs. If your MKV has AAC, AC3, or MP3 audio (common), FFmpeg must re-encode it. Select "Re-encode to Opus" or "Re-encode to Vorbis" instead of "Stream copy audio" unless you are certain the source audio is already Opus or Vorbis.

How long does MKV to WebM conversion take?

VP9 encoding is 3-5x slower than H.264. A 10-minute 1080p video can take 20-60 minutes on a modern CPU depending on the speed preset. Use "-deadline realtime" or "-speed 4" for faster encoding with slightly lower quality. AV1 with libaom-av1 is even slower; SVT-AV1 (libsvtav1) is a much faster alternative.

How do I install FFmpeg?

On macOS: run "brew install ffmpeg". On Windows: run "winget install ffmpeg" or download from ffmpeg.org. On Ubuntu/Debian: run "sudo apt install ffmpeg". On Fedora: run "sudo dnf install ffmpeg". Verify the installation with "ffmpeg -version".

Is my video file uploaded to this website?

No. This tool is a command generator — it builds an FFmpeg command string based on your settings. No file is selected, read, or uploaded. You run the generated command locally on your own machine.

Can I stream-copy MKV video into WebM without re-encoding?

Yes, if the MKV contains VP8, VP9, or AV1 video. Select "Stream copy (no re-encode)" for the video option. This is extremely fast and lossless. However, if the MKV contains H.264 or H.265 video, you cannot stream-copy it into WebM — those codecs are not supported by the WebM container.

MKV to WebM Converter

Generate a ready-to-run FFmpeg command to convert MKV video files to WebM format with VP8, VP9, or AV1 codec options.

FFmpeg Command Builder

Configure your options and get a ready-to-run FFmpeg command for MKV to WebM conversion.

Input filename

Include the path if the file is not in the current directory, e.g. ~/Downloads/video.mkv

Output filename

Video codec

Quality (CRF) — 33 (lower = better quality, larger file)

0 — lossless 33 — default 63 — worst

Encoding speed

Audio handling

Audio bitrate

Two-pass encoding

Runs FFmpeg twice for more accurate bitrate distribution. Produces two commands — run them in sequence. Only for VP8/VP9.

Generated FFmpeg Command

ffmpeg -i input.mkv -c:v libvpx-vp9 -crf 33 -b:v 0 -speed 4 -c:a libopus -b:a 128k output.webm

Click the command to select all, then copy. Run it in any terminal where FFmpeg is installed.

Command Breakdown

Flag	What it does

MKV vs. WebM — Format Comparison

Understand what you are converting and why

Feature	MKV	WebM
Origin	Open source, Matroska (2002)	Google / open source (2010)
Supported video codecs	H.264, H.265, VP9, AV1, and more	VP8, VP9, AV1 only
Supported audio codecs	AAC, MP3, Opus, Vorbis, AC3, FLAC	Opus, Vorbis only
Browser support	Limited (Chrome only, not Safari/iOS)	Universal — all modern browsers
Royalty status	Open (container); codecs may have fees	Fully royalty-free
Best use case	Local media, archival, Blu-ray rips	Web delivery, HTML5 video element
Subtitle support	Excellent (ASS, SSA, SRT, PGS)	WebVTT only

FFmpeg Installation

One command per platform — free and open source

# macOS (Homebrew)
brew install ffmpeg

# Windows (winget)
winget install ffmpeg

# Ubuntu / Debian
sudo apt install ffmpeg

# Fedora / RHEL
sudo dnf install ffmpeg

Command copied!

Summary

Generate a ready-to-run FFmpeg command to convert MKV video files to WebM format with VP8, VP9, or AV1 codec options.

How it works

Enter your MKV input filename (and path if needed) in the filename field.
Choose a target video codec: VP9 for the best quality-to-size ratio, VP8 for maximum compatibility, or AV1 for cutting-edge compression.
Set the quality level using the CRF slider — lower values produce higher quality and larger files.
Choose audio handling: Opus is the recommended codec for WebM and delivers excellent quality at low bitrates.
Optionally enable two-pass encoding for more precise bitrate control on VP8/VP9.
Click "Generate Command" to produce the FFmpeg command, then copy it and run it in any terminal with FFmpeg installed.

Use cases

Embed MKV movie or TV episode files on a website using the native HTML5 video element.
Serve open-source, royalty-free video without paying H.264 licensing fees.
Convert MKV recordings to WebM for upload to platforms that prefer open formats.
Reduce file size using VP9 or AV1 compression — often 40-50% smaller than H.264 at equivalent quality.
Prepare screencasts, tutorials, or course videos in a browser-native format.
Generate a WebM version alongside MP4 as a fallback pair for maximum browser compatibility.
Convert MKV files with Opus or Vorbis audio to WebM without re-encoding the audio stream.
Create WebM background videos for CSS animations or hero sections on websites.

Frequently Asked Questions

Last updated: 2026-09-22 · Reviewed by Nham Vu