How it works

Most online converters upload your file to a server. This one does not, and the difference is architectural rather than a policy promise. Here is exactly what happens when you convert a file.

The short version

When you drop a file onto a converter page, the file is handed to JavaScript running in your own browser tab. It is decoded into raw pixels, redrawn onto an in-memory canvas, and re-encoded into the target format — all using your device's processor. The result is written to a temporary in-memory URL that your browser turns into a download.

At no point does the file touch a network. There is no upload request to intercept, because there is no server-side component to receive one.

Step by step

1. You select a file. The browser hands our code a File reference. This is a local object — obtaining it does not transmit anything.
2. The file is decoded. For most formats the browser can do this itself. For HEIC, which browsers other than Safari cannot read, we load libheif compiled to WebAssembly and decode it locally.
3. Pixels are drawn to a canvas. This single stage is where resizing happens and where transparency is flattened if the target format has no alpha channel.
4. The canvas is re-encoded. The browser's built-in encoder produces the final JPG, PNG, or WebP, applying whatever quality setting you chose.
5. You download it. The result becomes a temporary object URL in memory. Closing the tab discards it.

What WebAssembly changed

Until fairly recently, converting a HEIC file genuinely did require a server, because no browser could decode the format and the libraries that could were native code. WebAssembly removed that constraint: it lets compiled C and C++ libraries run inside a browser tab at close to native speed. The same libheif that a server would have used now runs on your device instead.

That module is around two megabytes, so we load it only when you actually drop a HEIC file. Pages that do not need it never pay for it.

Verify it yourself

You do not have to take our word for any of this, and you should not have to. Load any converter page, then turn off your Wi-Fi or unplug your ethernet cable, and convert a file. It will work perfectly, because everything needed is already on your device.

For a closer look, open your browser's developer tools before converting and watch the Network tab. You will see the page and its scripts load, and then nothing carrying your file.

The honest limitations

Running on your device is a genuine trade, not a free win, and it is worth being straight about the costs:

• Memory is finite. A browser tab can only address so much memory. Very large files, or very large batches, can exhaust it — most often on phones and older machines.
• Speed depends on your hardware. A recent laptop will outrun a typical server. A six-year-old phone will not.
• Some formats we cannot write. We only offer conversions the browser can genuinely perform. Where a target format has no browser-side encoder, we do not list it rather than quietly uploading your file to work around the gap.