Client-side conversion runs entirely in your browser, so your file never leaves your device — while server-side conversion uploads your file to a company's servers to process it. For privacy, client-side wins decisively: there's no upload, no stored copy, and no third party handling your data.
What is client-side conversion?
The conversion runs on your own device, inside your browser, using WebAssembly and built-in browser APIs. Your file is opened locally and never sent anywhere. Because there's no server round-trip, it also works offline once the page has loaded, and there's no daily limit to meter.
What is server-side conversion?
Your file is uploaded to the company's servers, converted there, and returned as a download. This is the traditional model, and it's genuinely necessary for some heavy or specialised conversions — but it means a copy of your file leaves your device and lands on someone else's machine.
Which is more private?
Client-side, by a wide margin. If the file never uploads, it can't be stored, leaked, retained, or used to train anything. Server-side privacy depends entirely on the company's policies and follow-through; client-side privacy is structural — true by design, and something you can verify yourself in the Network tab.
When is server-side actually needed?
Being honest: some conversions still belong on a server — very large files, certain Office formats like PDF to Excel, or niche formats that need specialised engines. The goal isn't to pretend everything can be local; it's to keep everything that can be local, local, and be upfront about the rest.
How File Converts splits the two
Most File Converts tools are client-side and never upload — a live panel proves 0 bytes sent. The few that need a server (like PDF↔Office) are clearly labeled before you choose a file, and those files are deleted right after. See how it works, or read is it safe to use an online file converter.
How to tell which one you are using
- Open your browser's developer tools, go to the Network tab, and convert a file. A client-side tool shows no upload of your file; a server-side one shows a request carrying it
- Disconnect from the internet after the page has loaded. A genuinely local tool keeps working; a server-based one cannot
- Watch the timing: a 200 MB video that starts converting instantly was not uploaded, because the upload alone would take longer than that
When server-side is the honest answer
Some conversions genuinely cannot run in a browser. High-fidelity Office rendering needs a real Office-compatible layout engine, and PostScript rasterising needs Ghostscript, which is AGPL-licensed and belongs on a server rather than in a bundle shipped to every visitor. A tool that says it does everything locally, with no exceptions at all, is either very narrow in scope or not being straight with you.
What matters is that the split is disclosed before you pick a file, not discovered afterwards.