How to Check If a Website Uploads Your Files (5-Minute Test)

Plenty of online converters claim that your files are private. Some are, and some quietly send every file to a server you know nothing about. You do not have to take anyone’s word for it, because your browser records everything a page sends. This guide shows how to read that record in about five minutes, on any site, with no special software.

Why “uploaded” versus “processed locally” matters

A server-based converter copies your file to someone else’s computer, converts it there, and sends the result back. During that time you have no say over how long the copy is kept or who can reach it. A browser-based tool does the work inside your own tab, so the file stays on your device. We wrote more about the risks of careless converters in The Dangerous Truth About Free File Converters.

Tools that run locally exist. BuzzKit’s are built that way, using the browser’s Canvas API, WebAssembly builds of ffmpeg, and the pdf-lib and pdf.js libraries. But we would rather you could check us than believe us, so this test works on our tools as well as anyone else’s.

What you need

• A desktop browser: Chrome, Edge or Firefox all work.
• A test file big enough to make an upload obvious: 50 MB or more for video and PDF, 10 MB or more for images. Use a file with no sensitive content.
• Ideally a clean browser profile with extensions turned off, because extensions can add their own network traffic and muddy the results.

The test, step by step

1. Open the tool page, then open DevTools with F12 (or right-click, Inspect) and go to the Network tab.
2. Keep the record. In Chrome, tick Preserve log. In Firefox, enable Persist Logs in the Network Monitor settings. Then clear the list so you start from empty.
3. Load your file into the tool and run it. Download the result when it finishes.
4. Look for outgoing requests. Type method:POST in the filter box, then repeat with method:PUT. Uploads almost always use one of these. Click each request and check the size of what was sent.
5. Check the totals. In Chrome, the status bar at the bottom shows how much data was transferred in total. If you processed a 50 MB file and the page transferred 2 MB, the file did not go anywhere. If it transferred more than 50 MB, it very likely did.
6. Scan for other transports. Some sites upload in many small chunks, or over a WebSocket. Add up the sizes, and try the WS filter in Chrome.

The offline check

This is the most convincing step. After the page has fully loaded, open the throttling menu in the Network tab (it usually says No throttling) and choose Offline. Now run the tool again with a file.

• If it still produces a result, the work is happening on your device, because there is no connection to send the file through.
• If it fails, or shows requests in red, the page was trying to reach a server.

One fair caveat: tools that rely on a processing engine, such as ffmpeg or pdf.js, need to download it once. Load the tool and let it finish loading before you go offline.

How to read what you see

What this test cannot tell you

• It shows what happened in your browser on that day. A site can change its code later, so repeat the test occasionally for tools you rely on.
• You can see requests to third-party scripts, but not what happens inside them.
• Browser extensions can read page contents, which is why a clean profile matters.
• Forms that you fill in, such as a contact page, send the text you type. That is separate from the files you process.

Try it on BuzzKit

Pick any tool from the full tool list, such as the Video Compressor or Merge PDF, and run the steps above. If you see something that does not match what we say, tell us through the Contact page. We will look into it and correct this page if we got something wrong.

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