How to check whether an online tool uploads your files
Plenty of file converters say "secure" and "private". Almost none say where the work happens. There is a test that settles it in about thirty seconds, it needs nothing you do not already have, and it works on any website — including this one.
Why the words on the page do not tell you
"Secure" usually means the connection is encrypted — that the upload cannot be read in transit. It says nothing about whether there was an upload at all, or what happened at the other end. "We delete your files after an hour" is a policy, not a mechanism: it tells you the file arrived somewhere and that someone intends to remove it.
Neither claim is dishonest. They are just answers to a different question from the one you are asking, which is: did my document leave this machine?
The two designs
Server-side
Your file is sent to a machine you do not control, processed there, and the result comes back. This is why such sites have queues, upload size caps, and sign-up prompts once the file gets large.
Client-side
The page downloads code, and that code does the work on your own CPU. Nothing is sent. The limit is your device's memory rather than someone's pricing tier.
Both are legitimate engineering choices. Some jobs genuinely need a server — anything involving heavy machine learning, for example. But for resizing an image, merging PDFs or formatting JSON, a modern browser is entirely capable on its own, and the upload buys you nothing while costing you a copy of your file.
The test
The idea is simple: a file being uploaded has to travel, and travel is visible. Watch the network while you use the tool and look for a request about the size of your file.
- 1
Get a file that is big enough to spot
A few megabytes is ideal — a photo straight off a phone is perfect. A 30 KB file is too small; it hides in the noise of ordinary page traffic.
- 2
Open developer tools
F12 on Windows and Linux, or ⌥⌘I on a Mac. Every major browser has this built in; nothing to install.
- 3
Go to the Network tab and clear it
There is a clear button, usually a circle with a line through it. This removes the page-load requests so you only see what happens next.
- 4
Use the tool
Pick your file and run the conversion, compression, or whatever the site does.
- 5
Sort by size, largest first
The Size column header is clickable. This is the whole trick — you are looking for one big outgoing request.
Reading the result
A large POST appears, roughly your file's size
That is the upload. Click it and look at the Request tab — you will see the file's bytes. The site is server-side, whatever the marketing copy says.
Nothing large goes out
The work is local. You will still see requests — the page, its styles, fonts, analytics, advertising, and often a processing library of a few hundred kilobytes the first time you use a tool. Those are all downloads, and none of them contain your file.
One thing that trips people up: a library download can be large. Seeing a 500 KB request is not evidence of an upload. Check the direction and the type — a library arrives as JavaScript from the site's own domain or a CDN, while an upload leaves as a POST carrying your file's content type.
What this test cannot tell you
It shows what happened in the run you watched. It cannot promise the site will behave the same way next month, and no test performed in a browser can. A site could serve different code tomorrow.
It also does not mean a client-side site is sending nothing at all. Analytics and advertising scripts are still third-party requests, and they still reveal that someone visited. The meaningful distinction is between a site that knows you visited and a site that holds a copy of your contract.
Where it matters most
For a meme you were going to post anyway, none of this matters. It starts to matter with the files people quietly hand to converters every day: signed contracts, payslips, passport and ID scans, medical letters, and photographs — which carry EXIF metadata including the exact GPS coordinates where they were taken.
If you have never looked, the EXIF Viewer will show you what one of your own photos is carrying. It is usually more than people expect.
Run it on us
Every tool on this site is client-side, and the test above is the way to confirm it rather than believe it. How it works explains which browser APIs do the job, and is honest about the three things that genuinely are third-party requests here.