Convert video

How Video Converter works

The file is read with a FileReader, rewritten in memory and offered as a download. This is why large PDFs are limited by your device's RAM rather than by an upload size limit. A 200 MB scan never crosses the network, so the slowest part is your own disk.

That is the whole architecture: static files, one script, no upload endpoint to leak or log. Close the tab and the work is gone.

Container versus codec

An MP4 is a box; H.264 is what goes inside it. Converting a format can therefore mean one of two very different jobs: re-wrapping the same streams in a new box (seconds, no quality loss), or decoding every frame and encoding it again (slow, lossy, but necessary when the codec does not fit).

This tool re-encodes, because that is what actually fixes a video that will not play. The MKV to MP4 page offers the fast remux path when you know the codecs already fit.

What the settings mean

CRF is the quality dial: lower numbers keep more detail and produce bigger files. 20 is visually near-lossless, 23 is the sane default, 28 is for messaging apps that recompress anyway.

Audio is re-encoded to AAC at 128 kbit/s in MP4 and Vorbis in WebM. That is deliberate: copying audio into an incompatible container is the single most common cause of a silent video.

The honest limits of a browser encoder

WebAssembly runs roughly two to five times slower than native code, and a phone has one or two cores to spare. A one-minute 720p clip takes one to three minutes; a 4K clip will test your patience and your battery.

Memory is the hard ceiling: the decoded frames have to fit in the tab. That is why the tool refuses files over about 220 MB rather than crashing halfway through a conversion.

What the two output formats are good for

MP4 is H.264 at CRF 23 with AAC at 128 kbps and the index moved to the front of the file, so it starts playing before it has fully downloaded. That combination plays on Windows, macOS, Android, iOS, every browser, every TV and every social platform.

WebM is VP8 video with Vorbis audio at a lower bitrate. It is usually smaller for screen recordings and animation, and it is the better choice when the file only ever lives on a web page. It will not open in QuickTime or in most video editors.

Speed, heat and the 220 MB ceiling

Encoding runs on your CPU inside one tab, not on a GPU farm. Expect a 1080p clip to take roughly as long as it plays, sometimes longer on a thin laptop — and the fan will spin. Keeping the tab in the foreground matters: background tabs get throttled and the conversion crawls.

The first conversion downloads the 31 MB engine once; after that it is cached for the session. Files above 220 MB are refused because the tab would run out of memory partway through, which is a worse outcome than an honest no.

Inputs that will fail, and why that is correct

DRM-protected downloads from streaming services are encrypted on purpose; ffmpeg cannot decode them and no browser tool should try. Corrupted files with a broken index fail the same way. Neither is a bug to work around.

MOV from an iPhone usually converts cleanly, HEVC included, because the engine decodes H.265. A very high-bitrate 4K clip will convert but may take ten minutes or more — trim it first, then convert the part you need.

Good to know

• Works offline once loaded, including on flights and in locked-down networks.
• No file size limit beyond your device's own memory.
• Free to use, with no account and no usage counter.
• Results are produced locally, so they are not logged or shared with anyone.

Questions