Is my video uploaded anywhere?

No. The bytes are transferred into a Web Worker in your browser and never reach a network. The site’s automated tests assert this with network interception.

Why did my conversion grow the file?

Container conversion can add overhead when codecs are copied. The result reports the honest measured size — if it grew, you will see that plainly and can keep the original.

Does video compression work on my browser?

Inspection, conversion and trimming work wherever the site runs. Full re-encoding needs WebCodecs encoders (Chromium-family browsers are best) and is detected up front.

Convert MP4 video — locally, in your browser

Convert MP4 to WebM (and back), trim a clip to a time range, or inspect its tracks — all processed by your own browser via WebCodecs and modern container tooling.

Open the MP4 video converter tool Runs entirely in your browser — files never leave your device.

What this tool does

Video is the heaviest file type most people handle, and every conventional converter uploads it. This one does not: the container work happens in a Web Worker on your device, with real progress and honest size accounting.

Where the source and target containers share compatible codecs, conversion is a fast remux. Where re-encoding is required, your browser’s built-in WebCodecs encoders do the work locally — capability is detected up front and reported honestly.

How to convert an MP4 video without uploading it

Add the file. Drop your .mp4 into the home-page tool.
Choose convert or trim. Pick the target container (WebM) or a time range to keep.
Download locally. The converted video is saved straight back to your disk.

Supported operations

This list is generated from CompressKit's capability registry — only implemented and tested operations appear here.

Trim media

Cut a time range without re-encoding where the codec fits.
Convert format

Change the container (MP4↔WebM) — remux when codecs fit, transcode otherwise.
Inspect file details

Container and track metadata: duration, resolution, codecs.

Frequently asked questions

Is my video uploaded anywhere?: No. The bytes are transferred into a Web Worker in your browser and never reach a network. The site’s automated tests assert this with network interception.
Why did my conversion grow the file?: Container conversion can add overhead when codecs are copied. The result reports the honest measured size — if it grew, you will see that plainly and can keep the original.
Does video compression work on my browser?: Inspection, conversion and trimming work wherever the site runs. Full re-encoding needs WebCodecs encoders (Chromium-family browsers are best) and is detected up front.

Ready to try it?

The MP4 video converter tool runs on this site with no upload step — your file is processed by your own browser and saved back to your device.

Open the MP4 video converter tool