This site is published as Rules of Thumb at rulesofthumb.info (“we”, “us”). It is a personal / project publication of free reference calculators — not a registered company, and not a commercial data-brokering service.
There is a contact form for corrections, accessibility problems and privacy requests. There is no public email address: the form writes to this site’s own storage rather than a mailbox, and §2b sets out exactly what it keeps. Because we hold almost no personal data ourselves (see below), most formal rights requests about hosting-level logs need to be directed to Cloudflare as infrastructure provider. If a contact channel is added later, this policy will be updated with the address and date.
There is no account system and no sign-up. Calculator inputs are never submitted to a server. Specifically:
Each tool page has a Favourite star in the top controls. That saves the tool in local storage on your
device (for your home-page favourites and the search palette). The same click may also send a small request
containing only the tool identifier (for example mortgage) and an up or clear action, so we
can store an aggregate counter in Cloudflare KV for the popular tools ranking. We do
not store your name, email, or any calculator inputs with that request.
Soft rate-limiting may use your IP address transiently (same class of infrastructure data as section 3) to reduce abuse; we do not build a profile of voters.
If you send something through the contact form, we store what you typed: the category, your message, the page reference if you gave one, and a reply address only if you chose to enter one. The form works perfectly well without an address — an anonymous correction is just as useful.
Your IP address is not stored with the message. A non-reversible hash of it is used briefly, and only to limit how many submissions one sender can make in an hour; that counter expires within two hours and is never attached to what you wrote. There is no third-party form service involved.
Submissions are held in Cloudflare KV and deleted automatically after 180 days. Please don’t put financial details, account numbers, or anything else sensitive in the message — nothing about a calculator correction should need them.
Like any website, this site is served and protected by infrastructure that necessarily processes some technical data for every visitor, regardless of what you do on the page. This site is hosted on Cloudflare Pages, operated by Cloudflare, Inc. As part of delivering and securing the site, Cloudflare's network processes standard request data including your IP address, approximate location derived from it, browser and device information, referring page, and request timestamps. This is used to deliver content, detect and mitigate abuse (e.g. bot traffic, denial-of-service attempts), and produce aggregate, non-identifying traffic statistics. Cloudflare acts as our data processor for this purpose; their own privacy and data-processing practices are described at cloudflare.com/privacypolicy.
This site does not set first-party cookies, and does not currently run any analytics or advertising scripts. Cloudflare may set a small number of strictly-necessary cookies or similar identifiers purely for security purposes (such as distinguishing automated bot traffic from real visitors). These are exempt from consent requirements under UK PECR and the EU ePrivacy Directive because they are strictly necessary for the service to function securely, and contain no information that identifies you personally for marketing purposes.
If we introduce analytics, advertising, or any other non-essential cookie or tracking technology in future, we will update this policy and request your consent first via an on-page cookie banner, in line with UK and EU requirements.
This site uses your browser's local storage — a small amount of data kept on your own device, never sent to us as a tracking identifier — for: dark/light theme, metric/imperial unit preference, recent tools for search suggestions, pinned tools, per-tool favourites or history where a tool offers them (for example the unit converter), speed-test history on that tool, and which tools you Favourited (local pin list and whether this browser already counted toward the public tally).
This is fundamentally different from a cookie or a tracking identifier: it never leaves your browser, we cannot see it, it isn't tied to any account or identity, and it doesn't follow you across devices. You can clear it at any time through your browser's own site-data settings, and the site works normally without it — you'd just lose these saved preferences and suggestions between visits, with everything falling back to sensible defaults (unit preference falls back to a guess from your browser's language setting). We do not use local storage for analytics, advertising, or any purpose beyond these on-device conveniences.
Typefaces (Space Grotesk, IBM Plex Mono, Inter) are self-hosted on this site under
/assets/fonts/. Your browser does not contact Google Fonts or any other third-party
font CDN when you load a page — font files are served from the same origin as the rest of the site.
The Currency category on the Unit Converter is the one place
on this site that requests live data from elsewhere. Switching to that category (or typing a currency
conversion into its quick-convert box) causes your browser to request exchange rates directly from
api.frankfurter.dev, a free, open-source service publishing European Central Bank reference
rates. That request goes straight from your browser to Frankfurter's servers — never through this site —
and Frankfurter will receive your IP address and standard request metadata as part of it, independently of
this site and governed by their own terms. The result is cached in your browser's local storage for the rest
of the day so this only happens once per visit, not on every keystroke. No other page on this site makes any
request like this; everywhere else, the calculation runs entirely on your device with no outside data involved.
To the extent any personal data (such as an IP address in a server log) is processed as described above, we rely on legitimate interests (Article 6(1)(f) UK/EU GDPR) — namely, operating, securing, and maintaining the availability of the site. We do not process any special category data, and we do not carry out profiling or automated decision-making.
Under UK GDPR, the EU GDPR, and similar data protection laws, you have the right to:
In practice we hold almost no personal data ourselves beyond what Cloudflare processes automatically to serve the site. Hosting-level log requests should be directed to Cloudflare using their privacy channels. When a public contact address is published here, it will also accept privacy questions about this policy.
Cloudflare operates a global network, so technical request data may be processed in countries outside the UK or European Economic Area. Where this happens, Cloudflare maintains appropriate safeguards (including Standard Contractual Clauses and the UK International Data Transfer Addendum) — see Cloudflare's own compliance documentation for details.
If you are visiting from outside the UK/EU — including the United States, Canada, Brazil, Australia or elsewhere — the same principle applies: we do not sell, share, or monetise your personal data, and we collect the minimum technical data necessary to run a static, browser-based tool. If your local law (e.g. the California CCPA/CPRA, Brazil's LGPD, Canada's PIPEDA) grants you equivalent or additional rights, they apply to the limited data described above.
This site is a general-purpose engineering reference tool, not directed at children, and we do not knowingly collect personal data from children.
We may update this policy from time to time, for example if we add analytics, advertising, or a contact address. We'll update the “last updated” date above, and flag any material change on the site itself.
Use the contact form — it handles corrections to figures or sources, accessibility problems, and privacy requests. Choose Privacy request as the category and say what you need. There is no public email address; §2b sets out exactly what the form stores.
Requests concerning Cloudflare’s own hosting logs should also go to Cloudflare’s published privacy channels, since those logs are held by them as infrastructure provider rather than by us.