Every calculation on this site runs in your browser, and nothing you type is sent anywhere. This page says so in detail, including the parts that are less tidy.
The short version
Nothing you type here leaves your device. There are no accounts, no cookies, no local storage, no tracking pixels, no advertising, and no third-party scripts of any kind. The site has no way to identify you and does not try to.
One thing is counted, and it is counted by the host rather than by these pages: how many times each sheet was opened, roughly where from, and which page sent a reader here. No script, no cookie, nothing kept in your browser, nothing assembled into a profile. The section below says exactly what that is and why.
The rest is a claim you can check rather than take on trust: every page is a single HTML file, so view the source and read it.
Your numbers stay on your device
Every calculation on every sheet runs in your browser. The wing you are sizing, the weight you are budgeting, the pack you are choosing — none of it is transmitted, because there is no server here to transmit it to. The site is static files.
Turn your network off after the first visit and the sheets still work, which is the simplest demonstration of the point.
One thing worth knowing about links
Moving from one sheet to the next carries your figures in the web address, after the question mark. That is how the handoff works, and it is why Sheet 05 already knows your wing area when you arrive from Sheet 06.
It has two consequences. Those addresses go into your browser history like any other. And if you send someone a link copied from a sheet you have filled in, your figures travel with it. Delete everything from the question mark onwards to share a blank sheet.
What is stored on your device
- A cache of the pages themselves. A service worker keeps a copy of each sheet so the site works with no signal at the flying field. It holds the pages, the fonts and the logos, nothing about you, and only this site can read it. Clearing site data in your browser removes it.
- Nothing else. No cookies. No local storage. No session storage. No database. Your inputs are not remembered between visits — which is a real inconvenience, and it is the honest consequence of storing nothing.
Saving a design
Every sheet has a Save this design button. It writes a small JSON file to your downloads folder and does nothing else: the file never leaves your machine, it is not uploaded, and this site cannot read it back unless you hand it to the Load button yourself.
That is the whole answer to the inconvenience above. The alternative — remembering your inputs for you — would mean storing them in your browser, and the line above says this site does not do that. A file you can see, move, keep next to the build log and delete is a better answer than storage you cannot see, and it does not require you to trust this page about what it kept.
What the web host can see, and what is counted
Serving a page at all means a computer somewhere receives a request. Like every web host, ours — Cloudflare — records the usual technical details of that request: an IP address, a timestamp, which file was asked for, and the browser's own description of itself. That is the minimum a server needs to answer.
Those request records are counted, and this page used to say they were not. The host's own analytics are switched on, which means somebody here looks at how many times each sheet was opened, roughly where in the world from, and which page sent a reader here. It is aggregate and it is the host's, not ours: no script is added to these pages, no cookie is set, nothing is stored in your browser, and nothing about one visitor is assembled into a profile. If you turn your network off after the first visit, the sheets keep working and the host sees nothing at all.
Why count anything: a sheet nobody opens is a sheet not worth correcting, and the time that goes into this is finite. That is the whole of it. There is no advertising here, no audience being sold, and nothing to join your visit to.
This is the one part of the site that is not under the sheets' control, so it is written plainly rather than glossed over.
No third parties
The pages load nothing from any other domain. The typefaces are served from this site rather than from a font service, which is the usual quiet leak on sites like this one. The site sends a Content-Security-Policy header that names no external origin at all, so a browser would refuse to load one even if it appeared by mistake.
That is why the counting above adds nothing to the page. The usual way to measure a site is a script from somebody else's domain, and the header these pages are served with would refuse to run it. What is counted is counted by the host from the request itself, which is the one measurement that needs nothing from your browser. A check in the repository fails the build if the header is ever loosened while this page still claims otherwise.
Children
The site is suitable for anyone and collects nothing from anyone, so there is no age-related data question to answer.
If this changes
Adding anything that collects data — analytics, a comment box, a newsletter — means rewriting this page first, and dating the change. That has happened once. The host's own page counting was switched on in Rev D, and the two sections above were rewritten before it was rather than after: this page had said the request records were "not analysed for behaviour", and counting them is analysing them. Nothing else about what reaches your browser changed, and nothing else is planned.