Is this MP4 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 MP4 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.

Does converting MP4 to MKV re-encode the video?

No — it's a container swap. The H.264 video stream is moved directly into an MKV container without re-encoding. The process is fast and lossless.

Why would I want MKV over MP4?

MKV supports multiple audio tracks, subtitle tracks, and chapters in a single file. It's the preferred container for media players like VLC, Kodi, and Plex that support those features.

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.

MP4 to MKV — Free, Fast, No Upload

How to convert MP4 to MKV online

1
Drop your MP4 file
Drag and drop your MPEG-4 Part 14 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 MPEG-4 Part 14 → 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.

MP4 vs MKV: format overview

MP4

MPEG-4 Part 14

Moving Picture Experts Group · 2001

Compression: lossy
Transparency: No

✓ Universal compatibility across all platforms
✓ Excellent compression with H.264/H.265
✗ H.264 has royalty implications

MKV

Matroska Video

Matroska.org · 2002

Compression: lossy
Transparency: No

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

MP4 magic bytes: 00 00 00 xx 66 74 79 70

MKV magic bytes: 1A 45 DF A3

Why convert MP4 to MKV?

MP4 is the standard output format for cameras, smartphones, and video editors, but it has container-level limitations that become apparent in media management workflows. MP4 does not support more than one audio track natively in a way that media servers like Plex reliably detect, and it has limited support for embedded subtitle streams. When building a personal media library where you want multiple language audio tracks, forced subtitle support, and proper chapter navigation, MKV is the superior container.

Kodi, Plex, Jellyfin, and Emby all treat MKV as their primary video container and expose its metadata, subtitle tracks, and multiple audio streams through their interfaces. Archivists who collect foreign films with multiple dub tracks, home theater enthusiasts who want precise subtitle control, and developers building media applications find MKV gives them the flexibility that MP4 constrains. HandBrake, the most popular video conversion tool, outputs MKV natively alongside MP4.

For most MP4-to-MKV conversions, if the video codec in the MP4 is H.264 or H.265, the operation can be done as a lossless remux where the streams are simply transplanted into the new container. This is near-instantaneous and preserves 100 percent of the original video and audio quality. If external subtitle files or additional audio tracks are being merged during conversion, those are added to the MKV container simultaneously. The output file size is essentially identical to the source MP4, and playback quality is unchanged.

Quality & file size: MP4 to MKV

Typical file sizes: MP4 100–300 MB → MKV 200–800 MB.

Both MP4 and MKV use lossy compression. We transcode at high quality settings (equivalent to MKV's recommended web quality) to minimize generational loss.

Color depth: MP4 supports standard color, MKV supports standard color.

Transparency: MP4 does not support transparency. MKV does not support transparency — transparent areas become solid white.

Frequently asked questions

Privacy: how FormatDrop handles your files

Your MP4 files 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.