Does that online converter upload your files? Check it in 30 seconds
Most free converters send your file to their server first. Here are two quick tests, the browser Network tab and the airplane-mode test, that show whether a site uploads your passport, contract or video.
Updated
Short answer: most free online converters upload your file to their server, process it there and send the result back. A few, like Nibbo, do the work inside your browser, so the file never leaves your device. You don’t have to take anyone’s word for it. Two tests below take under a minute and need no technical skills.
Why it matters
When you shrink a scan of your passport for a visa form, convert a contract to PDF or compress a family video, the file can contain your name, address, ID number, signature and even the GPS location where a photo was taken. Once it is on someone else’s server, you depend on their promise to delete it. Many services say they delete files “after 1 to 24 hours”; that is still a copy of your document on a computer you don’t control.
For a meme, that’s fine. For a driver’s license, a bank statement, a medical report or a tax form, it’s worth checking first.
Test 1: the Network tab (computer, 30 seconds)
Every browser can show you what a page sends. Here is how on a computer:
- Open the converter site.
- Open the developer tools: press F12, or Ctrl+Shift+I on Windows and Linux, Cmd+Option+I on a Mac. In Safari, first turn on Settings → Advanced → Show features for web developers.
- Click the Network tab. Leave it open.
- Now pick your file and run the conversion.
- Watch the list. Sort by Size or look at the Transferred total at the bottom.
How to read it: an upload shows up as a request (usually POST or PUT) whose size is about the size of your file. A 40 MB video creates a 40 MB request. If you only see small requests, a few kilobytes for analytics, fonts or images of the page, the file stayed on your computer.
You can also type method:POST into the filter box in Chrome or Edge to show only requests that send data.
Test 2: airplane mode (phone or computer, 20 seconds)
This one works on a phone too.
- Open the converter page and wait until it has fully loaded.
- Turn on airplane mode or switch off Wi-Fi and mobile data.
- Pick your file and try to convert it.
If the conversion still works with no internet, the site cannot be uploading the file, because there is nowhere to upload it to. If you get an error such as “network error”, “upload failed” or a spinner that never ends, the site needs its server to process your file.
One caveat: a site could work offline and still send the file later, once you’re back online. The Network tab (test 1) is the test that settles it.
Signs a site uploads your files
- An “Uploading…” progress bar before any processing starts.
- A queue (“You are number 3 in line”) or a waiting time for big files: queues exist only where a server does the work.
- A daily limit or “2 free conversions per hour”: server time costs money, so the site rations it.
- The privacy policy says files are “deleted after X hours”: something can only be deleted from a server if it was stored there.
- A download link that works on another device: your file lives on their server.
None of these mean the service is bad. They mean your file left your device.
What we do at Nibbo
Nibbo’s tools run in your browser: video is compressed on your own graphics chip through WebCodecs, images and PDFs are processed with WebAssembly. Run both tests above on nibbo.org: in the Network tab you will see page assets and, if you accept cookies, small analytics requests, but nothing the size of your file. With airplane mode on, a tool page you opened once keeps working.
Questions
Is “we delete your files after 1 hour” safe enough?
It depends on the file. Deletion promises are hard to verify, and backups or logs can keep copies longer. For ID documents, contracts and medical files, a tool that never receives the file is the safer choice.
Does HTTPS mean my file isn’t uploaded?
No. HTTPS (the padlock) encrypts the file on its way to the server. It protects against someone on the same Wi-Fi, not against the server itself, which receives the file in full.
Can a browser extension or app do the same?
Desktop programs such as HandBrake or LibreOffice also process files locally. Browser extensions vary: some upload, some don’t. The Network tab test works for extensions’ web pages too.
What about my phone’s built-in converters?
Built-in tools (Photos, Files, the share sheet) work on the device. Third-party “converter” apps often upload; test them with airplane mode.