Do file converter apps upload your files?
Many of them do. The category splits cleanly into two architectures — apps that convert on your phone, and apps that are a polite front end for someone else's server — and an app's store listing rarely tells you which one you have installed. You can find out yourself in about twenty seconds, and you should.
Why so many of them upload
It is worth understanding the incentive before assuming malice, because the reason is boring: server-side conversion is dramatically easier to build.
One Linux box running ffmpeg, ImageMagick and LibreOffice can convert essentially every format that has ever existed. Write that once and you have an iPhone app, an Android app, and a website, all with the same enormous format list, none of them constrained by what the phone's operating system happens to support.
And the phone's constraints are real. iOS has no MP3 encoder and no WebP encoder. An on-device app has to solve those gaps itself, by compiling and shipping its own encoders. An upload-based app just... doesn't have the problem. The server has ffmpeg.
So the upload is usually a shortcut, not a conspiracy. That does not make it harmless — it just means the question to ask is "which kind is this?", not "who is out to get me?"
The Airplane Mode test
This is the whole answer, and it costs nothing.
Turn on Airplane Mode. Convert a file.
- It works. The conversion ran on your device. There is no ambiguity here — the bytes could not have gone anywhere, because there was nowhere for them to go.
- It hangs, spins, fails, or shows a network error. Your file was being sent to a server. There is no other explanation for why a local computation would need a network.
No privacy policy, marketing claim, App Store label or support-email reassurance is worth as much as this test, because this test cannot be written by a copywriter. Run it on any converter you already have installed. Run it on ours.
The tells, before you even open the app
If you want to guess in advance, these are reliable signals that an app converts on a server:
- A daily or monthly limit measured in files, minutes or megabytes. On-device conversion costs the developer nothing — your battery, your CPU, your problem. Nobody meters a computation they do not pay for. A quota almost always means someone is paying per gigabyte, and it is not you.
- An implausibly long format list. Hundreds of formats, including obscure CAD, ebook and legacy office types, is a server's format list, not a phone's. No mobile framework does that.
- The privacy policy talks about retention. Look for phrases like "files are deleted from our servers after 2 hours", "processing", or "sub-processors". That sentence is reassuring in tone and damning in substance: files can only be deleted from a server if they were on a server.
- A matching website. If the same brand runs a browser-based converter, the app is very often a wrapper around it.
- Progress that tracks your connection, not your phone. If a small file converts slowly on bad Wi-Fi and quickly on good Wi-Fi, you have your answer.
- It asks you to sign in to convert a file. A local computation does not need to know who you are.
App Store privacy labels help, but do not settle it. "Data Not Collected" is a meaningful signal and Apple does hold developers to their declarations. But the labels describe what a developer says they collect and link to you, which is a narrower question than "does my file transit your infrastructure". Treat a good label as encouraging, not as proof. The Airplane Mode test is proof.
Is uploading actually bad?
It depends entirely on the file, and pretending otherwise would be scaremongering.
Converting a meme, a stock photo, or a PDF of a public train timetable through an online service is fine. There is nothing to protect. The risk is genuinely zero and anyone telling you to panic about it is selling something.
The calculation changes when the file is yours: a payslip, a passport scan, a signed contract, a medical result, a photo of your children, a voice memo of a private conversation, an unreleased document from work. Those files carry consequences that a converted meme does not, and uploading them means a copy exists on hardware you do not control, in a jurisdiction you probably did not check, for a retention period you are taking on trust. We go through the specifics in is it safe to convert files online.
The honest framing is not "uploading is evil". It is "uploading is a decision, and you should be the one making it" — which requires knowing that it is happening.
The structural point
An app that converts on your device has no server. That is not a policy, a promise or a commitment that a future owner could quietly revise — it is an absence. There is no bucket to leak, no retention window to get wrong, no breach to disclose, no acquisition after which the terms change. The question of what they do with your files cannot arise, because they never have your files.
That property is not unique to any one app, and we are not going to pretend it is. Converting on a Mac with Preview, running ffmpeg on your laptop, using the Photos app or Shortcuts — all of these have the same property, all of them are free, and all of them are perfectly good answers. What matters is that you know which category the tool you are using falls into.
Where Convexy sits. Convexy converts on your iPhone or iPad and makes no network requests to convert anything — the only network calls it makes are to the App Store, to verify a purchase. It works in Airplane Mode. Please verify that rather than believe it.
Common questions
How can I tell for certain whether a converter app uploads my files?
Turn on Airplane Mode and convert a file. If the conversion completes, it ran on your device and nothing was uploaded. If it fails or hangs, the app needed a server. This is the only check that does not depend on trusting a written claim, and it works on any app.
Why would a converter app upload files at all?
Because it is far easier to build. A single server running ffmpeg and ImageMagick can convert every format on every platform, and it sidesteps the phone's own limitations — iOS, for instance, cannot encode MP3 or WebP at all. Server-side conversion is usually a shortcut, not a plot.
Do free converter apps sell my files?
We have no evidence that any particular app does, and we are not going to accuse anyone. What can be said from structure alone: once a file leaves your device you cannot verify what happens to it, you are relying on a retention policy you cannot audit, and free services have to make money somewhere — usually advertising and analytics. Judge the file, not the vibe: if you would not email it to a stranger, do not upload it to one.
Does the App Store "Data Not Collected" label mean an app converts on-device?
Not necessarily. The privacy label describes what data a developer declares it collects and links to your identity. A file that is uploaded, converted and deleted might not be declared as "collected" data at all. A clean label is a good sign, but it is not the same claim as "your file never leaves the phone".
Is it safe to use an online converter for something harmless?
Yes. For a meme, a stock image, a public document or anything you would happily post in a group chat, an online converter is fine and the risk is effectively zero. Reserve the caution for files that carry consequences: identity documents, contracts, medical records, work files, and photos of people who did not agree to be uploaded.
Does Convexy upload files?
No. Conversion runs entirely on your iPhone or iPad, and the app works in Airplane Mode. Its only network requests are to the App Store to verify a purchase. There is no conversion server, which means there is no file retention policy to trust.