Last month we ran a simple experiment: converted the same 40 MB video through twelve popular "free online converter" sites while watching the network panel, the retention policies, and the fine print. The pattern was consistent enough to describe as a system.
What the trace showed. Every service uploaded the full file before converting — expected, since they're server-side. Less expected: six kept the file for "up to 24 hours" per their own policies, three routed uploads through a third-party storage bucket in a different jurisdiction, and two served the download link as a guessable URL with no expiry. One retained the filename in an analytics payload. None of this is illegal; all of it is invisible until you look.
The alternative architecture. The same conversions can run inside your browser tab now. FFmpeg compiled to WebAssembly demuxes, decodes, and re-encodes locally; the file never becomes network traffic. We built our media tools that way — compression (https://toolfyra.com/video-compressor.html), conversion (https://toolfyra.com/video-converter.html), trimming (https://toolfyra.com/video-trimmer.html), merging (https://toolfyra.com/video-merger.html), audio extraction (https://toolfyra.com/video-to-audio.html) — and the privacy claim is verifiable by anyone: open DevTools, watch the Network tab, convert. Zero file bytes leave the device.
A checklist for choosing converters. If you must use a server-side tool (batch jobs, huge files), check four things before uploading anything sensitive: stated retention hours, whether the download link is authenticated, the storage jurisdiction, and whether the policy mentions filename logging. If any answer is "not stated," treat the upload as semi-public. For everyday one-file conversions, local tools remove the question entirely.
The bigger lesson. "Free" was never the price question — the price was always the file. Client-side conversion doesn't just save servers money; it moves the trust boundary from a policy page to your own machine, where it belongs. Full index of our on-device tools: https://toolfyra.com/sitemap.xml.
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