About MKV to MP4

You add up to 20 MKV files (1.5 GB each), and the tool converts them to MP4 one after another, showing per-file progress and marking how each was handled. Whenever the source codecs allow it — H.264, H.265/HEVC, or AV1 video with AAC, MP3, or AC-3 audio — the conversion is a remux: a container swap that copies the compressed streams untouched. A remux finishes in seconds even for gigabyte files and is mathematically incapable of losing quality, because no pixel is ever decoded or re-compressed.

Files that MP4 players can't accept as-is get the fallback path: VP8/VP9 video is re-encoded to H.264, and Opus, Vorbis, DTS, or FLAC audio is converted to AAC. Text subtitles (SRT, ASS) travel along as MP4 timed text; bitmap subtitles from Blu-rays and DVDs can't exist in MP4 and are dropped with a note.

The engine is FFmpeg — the same converter used across the video industry — compiled to WebAssembly and running in a Web Worker in your browser tab. The first conversion triggers a one-time ~31 MB engine download that your browser caches. From there, each file goes through four steps:

1. Mount. Your file is attached to FFmpeg's virtual filesystem as a read-only view — WORKERFS reads straight from disk, so even a 1.5 GB file doesn't need to be copied into memory.
2. Probe. FFmpeg reports the container's streams, and the tool decides the cheapest safe route: copy everything, copy video and convert audio, or re-encode fully.
3. Convert. The chosen FFmpeg command runs — for stream copies at roughly disk speed, for re-encodes at CPU speed. If a copy attempt fails on a quirky file, the tool automatically retries with the next-safest plan.
4. Deliver. The finished MP4 (with the faststart layout for instant web playback on files up to 512 MB) is handed back as a normal browser download.

The 1.5 GB per-file cap is honest engineering, not marketing: browsers cap WebAssembly memory at 2 GiB, and the finished MP4 must fit inside it before download.

Nothing is uploaded — the site has no endpoint that could receive a video in the first place. Mounting, probing, converting, and downloading all happen on your device; MKV to MP4 never sees a filename, a frame, or a byte of your video.

Nothing persists either. Converted files live in your tab's memory only until you download or remove them, and closing the tab releases everything. The only thing your browser keeps is the cached conversion engine itself, so your next visit skips the ~31 MB download — that cache contains our code, not your videos. Disconnect from the internet after the page loads and every conversion still works, which is the simplest proof that no server is involved.