How to check an online tool really doesn’t upload your files

Plenty of free tools promise that your files never leave your computer. Some of them mean it; others upload the file, process it on a server and delete it later. You do not have to take anyone’s word for it. Two checks, each a couple of minutes, show what a page actually sends — and they work on any site, this one included.

Check 1: watch the network

Every browser can show the requests a page makes. If a file is uploaded, it appears there as a request roughly the size of the file.

1. Open the tool, then open the developer tools: F12 on Windows and Linux, or Cmd+Option+I on a Mac. Choose the Network tab.
2. Press the clear button (a circle with a line through it) so the list is empty.
3. Add your file and run the tool as normal.
4. Look at the new requests. Sort by Size, and check any with the method POST or PUT.

A tool that works in the browser shows nothing the size of your file going out. You may still see small requests: analytics pings, adverts, or the page fetching code it needs. What matters is the direction and the size. A 4 MB PDF being sent somewhere shows up as a request of about 4 MB, usually a POST, and is hard to miss.

Check 2: turn off the connection

The simpler test needs no developer tools. If a tool really runs on your device, it does not need the internet once the page has loaded.

1. Open the tool and wait a few seconds for the page to finish loading.
2. Switch off Wi-Fi, or put the device in flight mode.
3. Add your file and run the tool.

If it still produces a result, the work happened on your device. If it fails with a network error, it needed a server — which is fine for some jobs, but it means your file, or something about it, was going to be sent.

On this site, most tools pass this test once the page has loaded, because the page saves what it needs as soon as you open it. Try it with Merge PDF, Compress Image or the Word Counter.

When a tool needs the internet, and why that can be fine

Some jobs cannot be done offline, and an honest tool says so. On Utilboxes these are the exceptions:

• Live lookups. The SSL checker, the Open Graph preview and the optional mail-server check in the email validator send only a domain name or address to this site’s server, because a browser is not allowed to make those checks itself. No file is involved.
• Reference data. The currency converter downloads the day’s exchange rates; the amount you type stays on your device.
• Language models. Image to Text and Voice to Text download a recognition model the first time. The model comes to you; your image or recording does not go anywhere.

The difference that matters is what travels and in which direction. Downloading code or a model is not the same as uploading your document.

What the checks cannot tell you

• They show what happened during your test, not what a site might do later. Repeat them if a tool changes.
• A browser extension can read pages and files independently of the site. Test sensitive work in a window with extensions disabled, or in a private window.
• Passing both checks means the file was not uploaded. It does not say anything about how well the tool did the job, so still check the result.

A quick checklist

• Network tab open and cleared before adding the file.
• No request close to your file’s size, and no unexplained POST.
• Tool still works with Wi-Fi off after the page has loaded.
• Any network use explained by the tool’s own page.