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.
Every calculation is written and run independently — usually as a short Python script — before the page is built, and every figure the finished page displays is compared back against it. Where a source publishes worked numbers, the tool is checked against those too.
This is not ceremony. It has repeatedly caught errors in the assumption rather than the arithmetic: a claim that a light sweat rate keeps you inside a 2% fluid loss over four hours (it does not), and a heating model whose ceiling temperature was treated as independent of the outside temperature when it cannot be. Both were wrong before any page existed. One planned tool was abandoned outright because its central claim could not be reproduced.
Twenty-nine automated checks run before anything is published, and they fail the build rather than print a warning. Among them: every tool must have a Rulebook rule and a cited source; every rule must say where it stops being true; every chart must have a name a screen reader can announce; no page may advertise a FAQ it does not show.
The strictest one watches the prose. Sentences describing what a tool does — its limits, what it deliberately does not model — are pinned to a fingerprint of that tool's own code. Change the code and the sentence is flagged to be read again. It covers the tool pages, the Rulebook, and all 249 rows of the Sources page. It exists because a page once carried a “does not model tax deductibility” note that was itself untrue.
A complete copy of the site is archived at every version — 311 so far — and the version history at the foot of each tool is produced by diffing those copies rather than from notes written afterwards, so it reflects what actually shipped. Site-wide housekeeping is filtered out; only changes to that tool appear.
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.
Every figure here is checkable, which means mistakes are findable — and several have been found and fixed. Corrections are welcome from anyone, with no account and no name required. There is a link at the foot of every tool and rule page that opens the form with the page already filled in.
What happens to one: the figure is re-derived from the source, the fix ships as its own version, and the reason is written into the changelog — including when the original error was ours. That public record is the point. A reference site that has never listed a correction is not one that has never made a mistake.
Rules of Thumb is published under its own name rather than an individual's. It 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.
Found something wrong? Tell us. The contact form takes corrections to figures and sources, accessibility problems, and privacy requests — no account, no mailing list, and an address is optional. Citations are listed 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.