Is this WEBM to MKV converter really free?

Yes. Up to 5 files at a time, no signup, no catch. The free tier adds a small watermark to image output — Pro ($9.99/mo or $49/yr) removes it, raises the file size limit to 500 MB, and unlocks unlimited batch conversion.

Are my files uploaded to a server?

No — and you can verify it yourself. Open DevTools → Network tab, then convert a file. You'll see zero upload requests. Everything runs right in your browser. We never see your files.

How long does conversion take?

Usually 1–3 seconds for a typical file. The very first conversion on a fresh page load may take 5–20 seconds while the converter engine loads — after that it's instant. Your CPU does all the work locally.

What is the file size limit?

Free: up to 10 MB per file, 5 files per batch. Pro: up to 500 MB per file, unlimited batch. If you're working with large videos or RAW photos, Pro is built for that.

Can I batch convert multiple WEBM files at once?

Yes — drop multiple files at once (or Ctrl/Cmd+click to select them) and they all convert in parallel. Free accounts handle up to 5 files per batch; Pro has no limit.

How large can my video files be?

Free accounts support files up to 10 MB — enough for short clips and GIFs. Pro raises that to 500 MB, which covers most 1080p videos. For files over 1 GB, a native app like HandBrake will be faster.

Will the video lose quality during conversion?

When only the container changes (e.g., MKV → MP4 where both use H.264), there's zero quality loss — it's a lossless rewrap. When the codec changes, we encode at CRF 23, which is near-transparent for most content.

Is WebM to MKV just a container change?

Often yes — WebM uses VP8/VP9 video and Opus audio, both of which are natively supported in MKV. In that case, conversion is a remux: no re-encoding, no quality loss.

Why does Plex sometimes prefer MKV over WebM?

Plex's media library management and metadata matching is optimised for MKV. WebM is treated as a web streaming format rather than a library file, and some Plex clients handle MKV more reliably.

How private is this converter?

Completely private — by design, not policy. There is no upload endpoint. Files are processed right inside your browser tab and disappear the moment you close it. We collect only anonymous page-view counts; your file content is never seen or stored.

WebM to MKV Converter — Free, Online, No Upload

How to convert WEBM to MKV online

1
Drop your WEBM file
Drag and drop your WebM Video Format file onto the converter, or click to browse your files. You can select up to 5 at once. Nothing leaves your device — conversion happens right here in the browser.
2
Hit Convert — it happens locally
Click Convert and watch it go. There's no upload, no server queue, no waiting. The converter runs WebM Video Format → Matroska Video entirely in your browser tab. Most files finish in 1–3 seconds.
3
Download your MKV
Your Matroska Video file is ready. Click Download, or grab a ZIP if you converted a batch. Close the tab and everything disappears — no copies kept anywhere.

WEBM vs MKV: format overview

WEBM

WebM Video Format

Google (On2 Technologies) · 2010

Compression: lossy
Transparency: No

✓ Royalty-free codec (VP8/VP9/AV1)
✓ Excellent web streaming support
✗ Not supported on iOS/Safari natively

MKV

Matroska Video

Matroska.org · 2002

Compression: lossy
Transparency: No

✓ Supports virtually any codec combination
✓ Multiple audio tracks and subtitles per file

WEBM magic bytes: 1A 45 DF A3

MKV magic bytes: 1A 45 DF A3

Why convert WEBM to MKV?

You have a WebM video from a web download, screen recorder, or browser-based tool, and you want to organize it in your Plex, Jellyfin, or Kodi media library. WebM is a web streaming format — Plex and Jellyfin treat it differently from library files, and some client devices and TV apps handle WebM inconsistently. MKV is the standard container for home media servers, with mature, reliable support across all Plex and Jellyfin clients.

MKV and WebM share codecs: VP9 video and Opus audio are native to both the WebM and MKV container formats. When your WebM already uses these codecs — the typical case for browser recordings and web downloads — conversion to MKV is a remux: the video and audio streams are moved to the new container without any re-encoding. This means zero quality loss and very fast processing, regardless of file size.

The practical result is that your video appears in Plex and Jellyfin as a proper MKV library file, with correct metadata matching, reliable thumbnail generation, and consistent playback across all client types including smart TVs, Roku, Fire TV, and Apple TV apps. MKV also supports richer metadata, embedded subtitles (SRT, ASS), and multiple audio tracks — features that WebM handles more awkwardly.

Quality & file size: WEBM to MKV

Typical file sizes: WEBM 50–200 MB → MKV 200–800 MB.

Both WEBM and MKV use lossy compression. We transcode at high quality settings (equivalent to MKV's recommended web quality) to minimize generational loss.

Color depth: WEBM supports standard color, MKV supports standard color.

Transparency: WEBM does not support transparency. MKV does not support transparency — transparent areas become solid white.

Frequently asked questions

Privacy: how FormatDrop handles your files

Your WEBMfiles are converted 100% inside your browser. They are never uploaded to our servers, never stored, and never seen by anyone other than you. This isn't a privacy policy claim — it's an architectural guarantee: our server has no endpoint that receives file bytes.