OneTaskBy Tim King · Updated
Video Converter
MP4, MOV, MKV and WebM to MP4 or WebM, No Upload
OneTask is free, with no sign-up, and built by one person.
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How the Video Converter works
This video converter turns an MP4, MOV, MKV or WebM file into an MP4 with H.264 video or a WebM with VP9 video. Drop the file in, pick the format and press Render. Tracks that already suit the new format are copied as they are, so nothing is lost. Everything else is re-encoded on your device with the browser's built-in video encoder. Nothing is uploaded.
Video converter: container and codec are different things
A video file has two layers. The container is the wrapper: MP4, MOV, MKV or WebM. Inside it sit the streams, each compressed with a codec. H.264, H.265 (HEVC), VP9 and AV1 are video codecs; AAC, Opus and MP3 are audio codecs. A player needs to understand both layers. That is why one MP4 plays everywhere and another refuses to open: the first holds H.264, the second holds H.265.
Converting can mean two jobs:
- Remuxing. The streams are moved into a new container without touching them. It takes seconds and keeps the quality exactly. An iPhone MOV holding H.264 and AAC becomes an MP4 this way.
- Re-encoding. The video is decoded frame by frame and compressed again with a different codec. It takes longer and loses a little quality. A WebM holding VP9 must be re-encoded to become an H.264 MP4.
The page picks the right job for each track. The results panel shows whether the video was copied or re-encoded.
Which format to choose
- MP4 (H.264) plays on every phone, computer, TV, editor and website made in the last fifteen years. Choose it when you do not know where the file will end up.
- WebM (VP9) is the open, royalty-free format used across the web. It suits embedding on a web page and usually gives a smaller file at the same quality as H.264, but some editors and older Apple devices will not open it.
How the conversion runs
The page uses WebCodecs, the browser feature that exposes the device's hardware video encoder and decoder to web pages. It reads the file with Mediabunny, an open-source media library, decodes only what has to change and writes the new file in memory. That runs as fast as your device allows rather than in real time, and the tab can sit in the background. A Cancel button appears while it works.
What it cannot do
- Subtitle tracks, chapters and extra audio tracks are left out. Only the main video and sound tracks are kept.
- Sound in a codec the browser cannot decode, such as AC-3 or DTS in some MKV files, is dropped and the page says so.
- The whole result is held in memory, so a phone may run out of room on a long 4K file.
FAQ
Does converting lose quality?
Not when the tracks are copied, which the results panel reports. When the video has to be re-encoded there is a small loss, much like saving a JPEG again. The page uses a high quality setting so the difference is hard to see.
Which browsers work?
Current Chrome and Edge on a computer, and Safari 17 or later. Other browsers work if they support WebCodecs video encoding. If yours cannot, the page says so as soon as it opens.
How big a file can I convert?
The limit is your device's memory, since the new file is built in memory. Files of a few gigabytes convert on most computers. On a phone, keep to clips of a few hundred megabytes.
Is my video uploaded?
No. The file is read and converted in this browser tab and never leaves your device. The page only needs the internet to load itself and the converter library, and the finished file is saved straight from memory to your downloads.
See also: all Audio & Video tools, Units, Images, Documents, Data & Code.
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