Rules of Thumb is a free, browser-based library of calculators and field heuristics — organised by discipline — with sources named on every serious claim. No account, no ads, no analytics today. Tools run in your browser.
A working reference for people who need numbers: engineers, students, DIY practitioners, and anyone checking an estimate before they dig into a full design package. Each tool is meant to be deep enough to be useful and honest enough about its limits.
The namesake page is The Rulebook — a collection of rules of thumb that each point at the calculator that does the job exactly. Estimate first; verify with the tool.
Formulas and tables come from standards, textbooks, agency datasets, or other citable references. The Sources page lists claims by area with the reference used.
Where a source publishes worked numbers, tools are checked against those before shipping. Where they do not, we use independent cross-checks (unit consistency, limit cases, second references).
Simple inputs up front; advanced options, schedules, and explanations further down. The goal is a usable first answer without hiding the detail when you need it.
Rules of Thumb is a personal project publication at rulesofthumb.info, governed by the laws of England and Wales (see Terms). It is not presented as a company brand.
There is no public email on the site yet. When one is added, it will appear on Privacy, Terms, and this page. For now, source corrections are invited via the note on Sources — keep a note of the tool URL, the figure you believe is wrong, and the better citation if you have one.
Tools deepen over time. High-level history lives on the Changelog.