WebM to MKV Converter

How to Convert WebM to MKV Online

1. Upload Your WebM File: Drag and drop or click "Add Files" to select a WebM video. Browser screen recordings, OBS clips, YouTube downloads, and Twitter/Discord WebM exports all work. Batch is supported.
2. Pick Video Codec and Quality Preset: Default is Very High, which keeps the VP9 video stream intact for a near-remux (fast, no quality loss). Switch the Video Codec dropdown to H.264 or H.265/HEVC if you need broader hardware-decoder compatibility, or AV1 for the smallest size at the cost of encode time.
3. Set Resolution, Bitrate, or Target File Size (Optional): Pick a Resolution Preset (4K, 1440p, 1080p, 720p, 480p), scale by percentage, or enter custom Width × Height. Cap output with Constant Bitrate / Variable Bitrate / Constant Quality (CRF) or set a Specific file size in MB.
4. Trim and Convert: Use the Trim section to pick a Time Range (start + duration), then click Convert. Files are uploaded over an encrypted connection, processed on our servers, and deleted automatically after a few hours — no sign-up, no watermark, never shared.

Why Convert WebM to MKV?

WebM is a stripped-down subset of the Matroska container that Google introduced in May 2010 for HTML5 video — restricted to VP8, VP9, and AV1 for video and Vorbis or Opus for audio so that browsers can play it without licensing concerns. MKV is the full Matroska container with no codec restrictions, full chapter and subtitle support, multiple audio tracks, and metadata for things like cover art. Most WebM→MKV jobs are a near-remux: the underlying stream is already Matroska-compatible, so the conversion just rewraps it into the richer container.

• Plex, Jellyfin, Kodi, and Emby libraries — these media servers prefer MKV because they can carry multiple audio dubs, forced/full subtitles, and chapter markers in one file. WebM has no subtitle support at all.
• Adding SRT or ASS subtitles — MKV embeds soft subtitle tracks (selectable, toggleable). Convert the WebM to MKV first, then mux subtitles in with a tool like MKVToolNix.
• Joining multiple audio tracks — original audio + commentary + alternate language can live side-by-side in MKV; the player selects at runtime. WebM only allows one audio track.
• Lossless archival of YouTube / browser recordings — keep the Codec on its current setting and the conversion is a stream copy: same VP9 video, same Opus audio, zero re-encoding, zero quality loss, completes in seconds even for hour-long clips.
• Playing on TVs, NAS boxes, and set-top players — devices like LG / Samsung TVs and Nvidia Shield index MKV natively but skip WebM files. Same bytes, different wrapper, suddenly playable.
• Editing in DaVinci Resolve, Premiere, Final Cut — these NLEs reject WebM but accept MKV (especially when the inner stream is H.264 or H.265). Converting also lets you swap codecs in the same step.

WebM vs MKV — Container Comparison

Codec & Quality Preset Cheat Sheet

Frequently Asked Questions

Is converting WebM to MKV lossless?

Yes, if you keep the existing codec. WebM is technically a subset of MKV — both use the EBML container under the hood — so when you leave the Video Codec on its current value and the Quality Preset on Very High, the converter performs a stream copy: it rewraps the same VP9 or AV1 video and Opus audio into the larger MKV container without re-encoding. Bytes in the video/audio streams are identical; only the wrapper changes. Re-encoding only happens when you switch the codec (e.g., to H.264) or change resolution/bitrate.

Why would I convert if WebM already plays in my browser?

Because MKV plays in many things WebM doesn't: Plex, Jellyfin, Kodi, Emby, VLC's TV interfaces, Nvidia Shield, Apple TV via Infuse, most modern smart TVs (LG, Samsung, Sony), Nintendo Switch homebrew players, and editing software like DaVinci Resolve or Premiere Pro. WebM is great for the open web; MKV is the universal container for everything that isn't a browser. The conversion also unlocks subtitles, multiple audio tracks, and chapter markers — none of which WebM supports.

Will my YouTube WebM download convert losslessly?

Almost always yes. YouTube serves VP9 video and Opus audio inside a WebM wrapper. MKV accepts both codecs natively, so the converter rewraps without touching the streams. A 1080p, 1-hour download that took 4 minutes to fetch will convert in under a minute and produce a file within a few hundred KB of the original. If you change the codec to H.264 for AppleTV / older-TV compatibility, expect a real encode pass that scales with clip length.

Should I pick H.264, H.265, or AV1 when re-encoding?

H.264 — universal hardware support back to 2007 GPUs and most TVs / phones. Largest files. H.265/HEVC — ~40-50% smaller than H.264 at the same quality per MSU/Netflix benchmarks; supported by iOS 11+, Apple Silicon, Nvidia 900-series and newer, most 2017+ TVs. AV1 — smallest of the three, royalty-free, but slowest to encode and requires recent decoders (iPhone 15 Pro and later, Android with AV1 hardware, modern Chrome/Firefox). For a long-term home library: H.265. For a clip you'll edit in Premiere: H.264.

Can I add SRT subtitles after converting?

Yes, but xconvert only handles the container conversion itself. To add subtitles, convert WebM→MKV here, then open the MKV in MKVToolNix (free, all platforms) and drag your .srt file in as an additional track. Player will let you toggle it on/off. WebM has no subtitle support, so this two-step flow is the standard workflow.

How do I keep file size reasonable when re-encoding?

If you're switching off VP9, your two main levers are Constant Quality (CRF) and Specific file size. For H.264, CRF 18-23 is the visually-lossless to high-quality range (lower = bigger and better; default 23). For H.265, CRF 22-28 hits the same band. If you need an exact size, use Specific file size in MB — the encoder runs a two-pass average bitrate calculation to land near the target. Resolution is the other lever: dropping 4K to 1080p halves file size before the codec even starts.

Does the conversion preserve metadata, chapters, and multiple audio tracks?

WebM normally only carries one audio track and no chapters, so there's typically nothing extra to preserve. If your WebM was unusual and had embedded metadata, the conversion keeps it. The real value of MKV is what you can add after: chapters via MKVToolNix, additional audio tracks (commentary, dubs), subtitle tracks, cover art, and rich metadata that Plex/Jellyfin will index.

What's the largest WebM file I can convert?

Files process on our servers, so the practical cap is upload size and connection speed. Multi-GB 4K WebM downloads convert fine. For lossless rewrapping (codec unchanged), memory use is minimal and even very large files work. For full re-encodes to H.265 or AV1, expect long processing times that scale with codec complexity and clip duration.

What if I need MP4 instead of MKV?

MP4 is the right pick when you need to upload to social media, embed in PowerPoint or Keynote, or play on iOS without third-party apps. MP4 supports H.264, H.265, and AV1 but not VP9 or Opus, so the conversion always re-encodes audio. See WebM to MP4 for that flow, or MKV to MP4 if you've already converted and want a second pass.