How to check whether a website uploads your files

Open the converter page and wait until it has fully loaded. Then turn off Wi-Fi or switch on airplane mode and convert a file. A tool that works in the browser keeps working, because everything it needs is already on your device. A tool that uploads files fails immediately or hangs on "uploading".

Some browser-based tools download extra components the first time you use a feature (for example a video engine). If the test fails, reconnect, use the feature once, and try again offline.

On a computer, press F12 (or Ctrl+Shift+I, Cmd+Option+I on Mac) to open developer tools and choose the Network tab. Tick "Preserve log", then convert a file. Every request the page makes appears as a row.

Look at the Method and Size columns. An upload shows up as a POST or PUT request whose size is roughly the size of your file. Small requests to ad networks or analytics are normal on many sites; a multi-megabyte request to the site's own server while you convert is your file leaving the device.

Phone browsers don't have developer tools, but the airplane-mode test works the same way. Load the page, switch on airplane mode, then pick a file and convert it. If you want more detail, you can connect an Android phone to a computer and use Chrome's remote debugging (chrome://inspect) to see the phone's Network tab on the desktop.

A progress bar labelled "uploading" before any work starts. Waiting times that grow with your internet speed rather than your device speed. A download link that comes from a different domain, or that expires after a few hours. A message that your file "will be deleted from our servers" — a promise you only need if it was there in the first place.

None of these are proof on their own, but together they tell you a server is doing the work. The Network tab gives the definitive answer.

No large outgoing request means the file was processed locally. That still says nothing about tracking scripts, so read the privacy policy for what the site does with page visits, cookies and ads.

If a site does upload, it isn't automatically bad — many good services need a server for heavy work. What matters is that they say so honestly and tell you how long files are kept.