Free online MKV to WEBM converter

Some browsers, video players, and publishing systems require a WebM file rather than an MKV file. Conversion can also create a smaller browser-ready copy while leaving the original MKV unchanged for archiving.

What the MKV format is

MKV, or Matroska Video, is an open multimedia container that can hold video, multiple audio tracks, subtitles, chapters, menus, attachments, and metadata in one file. It does not specify a single codec; MKV files commonly contain H.264, H.265/HEVC, VP9, AV1, or other video formats.

MKV is used for screen recordings, Blu-ray backups, downloaded video, high-quality archives, and files containing several language or subtitle tracks. Software such as OBS Studio, MakeMKV, HandBrake, FFmpeg, and video editors can create or process MKV files.

What the WEBM format is

WebM is a web-oriented multimedia container intended mainly for HTML5 video and browser playback. Its standard codecs are VP8, VP9, or AV1 for video and Opus or Vorbis for audio.

WebM has a narrower feature and codec range than MKV, but browser and web-application support is more predictable. Its simpler web-focused structure can also avoid unnecessary container features, although it does not automatically make every source file smaller.

How to convert with FFmpeg

FFmpeg provides the most control over codecs, streams, quality, and scaling. Install a current FFmpeg build, open a terminal in the folder containing the MKV file, and run:

ffmpeg -i input.mkv -map 0:v:0 -map 0:a:0? -c:v libvpx-vp9 -crf 30 -b:v 0 -c:a libopus -b:a 128k output.webm

This selects the first video stream and the first audio stream if one exists, then encodes video as VP9 and audio as Opus. Replace input.mkv and output.webm with the actual filenames. With VP9 constant-quality encoding, a lower -crf value produces higher quality and a larger file; values around 24–35 are practical starting points. The best setting depends on the source and intended playback size.

For a smaller frame size, add a filter such as -vf scale=1280:-2. For faster but less efficient VP9 encoding, add -deadline good; slower settings generally improve compression efficiency. To use AV1 instead, replace libvpx-vp9 with an available AV1 encoder such as libaom-av1, but expect substantially longer encoding times and less support on older devices.

When remuxing is enough

If the MKV already contains WebM-compatible streams, FFmpeg can change the container without re-encoding:

ffmpeg -i input.mkv -map 0:v:0 -map 0:a:0? -c copy output.webm

Remuxing is fast and does not reduce quality, but it works only when every selected stream is valid in WebM. H.264, H.265/HEVC, AAC, DTS, and many subtitle formats generally require re-encoding, removal, or a different output container. Check the source codecs first with ffprobe input.mkv.

How to convert without installing software

You can convert the file for free on 101convert.com. Upload the MKV, select WEBM as the output format, start the conversion, and download the resulting file after processing finishes. Do not upload confidential material, and ensure that you have permission to process copyrighted video.

Quality and compatibility limits

Converting H.264 or H.265 to VP9 or AV1 is a lossy re-encode. The output cannot restore detail already lost in the source, and repeated conversions can reduce quality further. Keep the original MKV and inspect the WebM at its intended playback size before replacing it.

WebM may not preserve every MKV feature. Multiple audio tracks, chapters, attachments, menus, image-based subtitles, and unusual metadata can be dropped or unsupported. Add explicit -map options when selecting additional compatible streams, then verify the result with ffprobe output.webm.

VP9 with Opus is usually the safest general-purpose choice for current browsers. AV1 can produce a smaller file at comparable quality, but encoding is slower and playback may be limited by older browsers, operating systems, or hardware.