Compress MKV
Convert MKV to a much smaller MP4 with FFmpeg — running locally in your browser.
or drop files here
Maximum file size: 200MB per file
The encoder picks the highest bitrate that fits your target.
Related Tools
Compress AVI
Compress AVI videos in your browser with FFmpeg WebAssembly. Convert AVI to a smaller MP4 locally — free, private, no upload.
Compress MP4
Reduce MP4 video file size by up to 80% while maintaining video quality. Free online video compressor - 100% browser-based, no upload required.
Compress Video by Size Limit
Compress videos to platform size limits like 8MB, 10MB, 25MB, 50MB or custom targets in your browser.
Compress WebM
Reduce WebM file size by up to 80% while maintaining video quality. Free online WebM compressor - 100% browser-based, no upload required.
About MKV Compression
MKV is a flexible container that often carries very high bitrate video, which is why MKV files are so large. This tool runs a full FFmpeg build compiled to WebAssembly inside your browser: it decodes your MKV, re-encodes the stream as modern H.264 MP4, and hands you a much smaller file — without uploading anything to a server. Old camcorder clips and archive footage finally fit into email and chat limits. Whether you are optimizing assets for a US-hosted site on AWS, Cloudflare, or Vercel, trimming email attachments for Gmail and Outlook, or submitting documents to federal portals, this tool keeps your files lean and your workflow fast — all without uploading anything to a server.
When to Compress MKV Files
Large Rips
Slim down large MKV movies and episodes for mobile and chat
Email Attachments
Meet attachment limits that raw MKV always exceeds
Sharing
Send clips in MP4 that every player and platform accepts
Storage Saving
Reclaim disk space from legacy MKV collections
How to Compress an MKV File Online
Upload your MKV files
Choose one or more `.mkv` videos from your device.
Pick mode, quality and resolution
Compress by quality, or enter a target size in MB and let the encoder fit it.
Compress and download
FFmpeg runs locally in your browser; download the smaller MP4 files when done.
Frequently Asked Questions
Why is MKV converted to MP4?
MKV usually wraps high-bitrate streams that exceed every chat and email limit. Re-encoding to H.264 MP4 typically cuts size by 50–80% while improving compatibility with phones, browsers and chat apps.
Does the file get uploaded anywhere?
No. The FFmpeg engine itself is downloaded once from a CDN and cached by your browser; your video files are processed entirely in local memory.
Why is the first compression slower?
The first run fetches the ~31MB WebAssembly engine. Afterwards it is cached and every compression starts immediately.
How big can input files be?
Up to about 200MB depending on your device memory, since everything runs in-browser. For larger files, split them first or use the 500MB-target MP4 tools on an already-converted copy.