What "runs in your browser" actually means
When a tool says it works entirely in your browser, it means the whole job — reading the file, transforming it, building the output — happens using JavaScript on your own device, through APIs like the File API and Canvas that have been built into browsers for years. Your file gets read into your computer's memory, gets processed right there, and the result comes back as a download. None of that needs to talk to a server, because there's nothing a server needs to do.
Compare that to how most "free" converters actually work: you pick a file, it gets uploaded somewhere, something runs on their servers, and you download the result back. That upload step is the entire difference, and it's the part most people never think to ask about.
Why the upload step matters
Uploading a file, even briefly, means it existed on someone else's server at some point — even if it's deleted five minutes later, even if the privacy policy says all the right things. You're trusting that the deletion actually happens, that nothing got logged or cached along the way, that a bug or a breach doesn't expose it later. For a random meme, none of that matters. For an ID scan, a contract, a screenshot with something private in it, it's a fair question to ask.
In-browser tools skip the whole trust question. There's no server in the loop to trust, because there's no server in the loop, period.
How to actually check, instead of taking a site's word for it
Open your browser's developer tools (F12 on most browsers, or right-click → Inspect), switch to the Network tab, and then use the tool normally — drop in a file, run the conversion. If it's genuinely local, you won't see any request carrying your file's data anywhere. Watch closely though: plenty of sites still send small bits of real traffic (page views, button clicks) that have nothing to do with your file, so you're specifically looking for a request whose body actually contains your file's bytes or filename, not just any network activity at all.
Where this applies on this site
Every tool here works this way: Image → PDF, the calculator, the JSON formatter, all of it. Nothing you drop into any of these gets uploaded anywhere; the JavaScript running in your tab does the entire job itself. You're welcome to pop open the Network tab and check while using any of them. That's a better way to trust a claim like this than just reading it on a page, ours included.