How it works

Image, video, audio, document, archive and font formats are handled by established open source libraries: FFmpeg for anything with a timeline, libjpeg, libpng, libwebp and libavif for stills, pdf.js and pdf-lib for PDF, and so on. Those libraries are compiled to WebAssembly, which browsers execute at close to native speed.

When you choose a file, the browser reads it from disk into memory in this tab. The WebAssembly module decodes it there, re-encodes it into the target format, and hands back bytes that become a download. The file never becomes an HTTP request. You can confirm that in your browser's network panel, or by disconnecting from the network and converting anyway.

Heavy work happens in a web worker, so the page stays responsive while a large video transcodes. Progress you see is the real state of that worker rather than an animation.

There is no upload wait. On a large file, transferring it to a server and back usually costs more time than the conversion itself, and none of that applies here. Conversion speed is bounded by your own processor, and a batch of files is processed one after another without a queue shared with strangers.

There is no retention question. A server-side converter has to store your file at least briefly, which means a retention policy, an access control model and a breach surface. Doing the work locally removes all three rather than documenting them.

It works offline. After the first visit, the engine is cached by the browser, so the page keeps working with the network switched off.

The practical file size ceiling is your device memory, since the input and the output both have to fit in the tab. Files up to a few hundred megabytes are usually fine on a desktop machine; phones and older laptops run out sooner.

Some conversions are inherently lossy or approximate, and the pages for those conversions say so rather than implying a perfect result. A few proprietary formats can be read but not written, which is why the site generates a conversion page in one direction and not the other.

The first run of a given engine downloads its WebAssembly module, which can be several megabytes for video. That happens once, and the browser caches it afterwards.

Converts.to is built and operated by CROapps Oy, a limited company registered in Finland.

There is no account system, no paid tier and no advertising, and the site collects no analytics. See the privacy policy for what that means in practice, and the terms of use for the conditions it is offered under.