Loads its engine on first use, so there's no offline download — but it still runs privately in your browser, nothing uploaded.
About this tool
Convert a video between the MP4 and WebM containers without uploading it anywhere. Pick a target format and, optionally, a video bitrate to control the file size and quality.
Everything runs in your browser using the WebCodecs API and the mediabunny library, so your file never leaves your device. When the source codec is already compatible with the target container, the tool remuxes — repackaging the streams losslessly with no quality change. When it isn't (or when you set a bitrate), it re-encodes the video to H.264 for MP4 or VP9 for WebM using your browser's WebCodecs encoders.
Re-encoding depends on the codecs your browser and hardware provide; most Chromium-based browsers bundle software H.264 and VP9 encoders. If an encoder isn't available, the conversion will report a clear message rather than fail silently.
Frequently asked questions
Which conversions are supported?
MP4 (H.264/AAC) to WebM (VP9/Opus) and WebM back to MP4. Leave the bitrate blank to remux when possible, or set one to force a re-encode at a target quality.
Is the conversion lossless?
Only when it remuxes — if the source video codec already fits the target container, the streams are copied without quality loss. Changing container across incompatible codecs, or setting a bitrate, re-encodes the video and is not lossless.
Is my video uploaded to a server?
No. All demuxing, remuxing and encoding happen locally in your browser; the file never leaves your device and there is no sign-up.
Why did it report a codec problem?
Re-encoding uses your browser's WebCodecs encoders (H.264 or VP9). If your browser or device doesn't provide the needed encoder, the conversion can't run — try the other target format or a browser that bundles software encoders, like current Chrome or Edge.
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