No. The Matter Size Calculator runs entirely in your browser. Your files are read on your own computer to count pages and measure audio/video length: nothing is uploaded, stored on our servers, or sent anywhere.
This is deliberate: briefs of evidence are confidential and privileged, so the tool is built so the material never leaves your device. When you start a scan you’ll see a quick confirmation reminding you of this. Because everything happens locally, the tool keeps working even with no internet connection once the page has loaded.