How to tell if an online tool uploads your files
"Free online converter" can mean two very different things: a page that does the work in your browser, or a page that sends your file to a stranger's server and sends a result back. The page rarely tells you which. These three checks do — and they work on any site, not just ours.
Why it matters
When a tool uploads your file, a copy exists on a machine you do not control: it can be logged, kept in backups, read by staff, or exposed in a breach — and none of that is visible to you. A tool that processes the file on your own device has no such copy to protect. For a tax form, a contract, a medical scan or a photo with location data, that difference is the whole point.
- 1
The airplane-mode test
Simplest · 20 seconds · works on phones
Load the tool page, then switch on airplane mode (or unplug your network). Now use the tool on a real file. If it finishes the job with no connection, the work happened on your device — there was nowhere to upload to. If it fails or hangs the moment you go offline, it depends on a server.
The page itself needs a connection to load the first time. Go offline only after the tool is on screen.
- 2
The Network-tab test
Definitive · 1 minute · desktop browsers
Open your browser's developer tools (press F12, or right-click → Inspect) and pick the Network tab. Clear it, then use the tool with a file of a few megabytes. Look at what appears: a local tool shows only small requests such as fonts, scripts or a tiny analytics ping. An uploading tool shows a POST or PUT request roughly the size of your file, usually to a path like /upload, /api/convert or a cloud-storage domain.
Sort the list by Size. Your file will be the largest row by far if it left the page.
- 3
The size-and-speed test
Heuristic · no tools needed
Try a large file. A local tool processes a 50 MB image or PDF about as fast as your computer can, and its only limit is your device's memory. An uploading tool takes time proportional to your upload bandwidth, often refuses files above a fixed cap, and may offer a paid plan to raise it. Notices such as "your file will be deleted after one hour" are another tell: a file can only be deleted from a server if it was sent there.
This test hints; the first two tests prove.
Reading the signs
| What you observe | Runs locally | Uploads to a server |
|---|---|---|
| Keeps working in airplane mode | Local | Server |
| A request the size of your file appears in the Network tab | No | Yes |
| Hard file-size cap with a paid tier to raise it | Rare | Common |
| "Files are deleted after N hours" notice | Not needed | Common |
| Watermark on the free-tier output | Rare | Common |
| Progress bar that tracks your upload speed | No | Yes |
A promise like "we never store your files" is a policy. Local processing is an architecture — and unlike a policy, you can verify it in twenty seconds.
Try it on this site
Every Momo Tools utility is built to pass these tests: open the Image Compressor or the PDF Redact tool, switch on airplane mode and run it. The why-local page explains the architecture, and the offline tools list groups every tool that handles files, passwords, IDs or pasted text.
FAQ
Is HTTPS enough to keep my file private?
No. HTTPS encrypts the file while it travels, but the server at the other end decrypts and reads it — that is how it processes your file. HTTPS protects you from eavesdroppers on the network, not from the service itself. Only local processing keeps the file on your device.
A tool says it deletes my files after an hour. Is that safe?
Safer than keeping them, but you cannot verify a deletion promise, and the file still passed through and was readable on someone else's machine, including backups and logs you never see. A tool that never uploads has nothing to delete.
Can a website claim to run locally but secretly upload anyway?
Yes — which is exactly why this guide asks you to test instead of trust. The airplane-mode and Network-tab checks observe what the page actually does, so a false claim shows up as a failed test.
Can I do this on a phone?
The airplane-mode test works perfectly on phones: load the tool, enable airplane mode, then use it. The Network-tab test needs a desktop browser, or a phone connected to a desktop for remote debugging.
What about browser extensions?
Extensions run with their own permissions and can read page content regardless of how a website is built. A locally-running tool cannot protect you from a malicious extension, so keep your extensions to ones you trust — that is a separate line of defence from choosing local tools.