How these calculators are built and checked

260 calculators, 260 of them carrying a citation to the standard their formula comes from — the schema rejects a calculator that has none, so that number is not allowed to drift. Last review pass: 8 September 2026.

The number is computed, not written

A language model chooses which calculator answers your question and reads the numbers out of it. It never does the arithmetic. Every figure on this site comes from a fixed evaluator running a formula that was written down in advance, which is why each result can show you its derivation step by step instead of asking you to trust it.

That distinction matters most when a question has no calculator yet. The chat can write one, but the formula it writes is checked against a whitelist and run by the same evaluator as everything else — and it stays unpublished until a person has reviewed it.

Sources are required, not decorative

Each calculator names where its formula comes from: a standards body, a regulator, or peer-reviewed work. NIST for units, the ICC and AWC for construction, the CDC and NIH for health, the IRS and CFPB for money. The build fails if a calculator has no source, so this is an invariant rather than an aspiration.

Conversion factors are entered by hand from the defining standard rather than taken from a units library. An acre is exactly 43,560 square feet; a library that models the US survey foot returns 43,560.0385, and on a site whose claim is exactness that is not a rounding we are willing to make.

What the build actually verifies

Every calculator carries a worked example with a known answer, and every one of them is re-run on each build. The mortgage schedule is checked twice by independent routes — summing 360 rows of interest, and evaluating the closed-form total — and the build fails if they disagree by more than fifty cents. Conversion factors are pinned to exact integers where the standard defines one.

This is the part that is automatic, and it is narrow: it proves the arithmetic still does what it did, not that the formula is the right one for your situation. Choosing the formula, and checking that its source still says what it said, is done by a person, and the date on each page is when that last happened.

Prices are ranges, not quotes

Where a calculator or guide shows costs, they are wide indicative ranges carrying the year they were gathered. Materials pricing moves regionally and seasonally; treat these as a sanity check on a quote you have been given, never as a substitute for one.

Health and money

BMI, body fat, calorie and heart-rate calculators implement published population formulas. Population formulas describe groups and are wrong about individuals in predictable ways — they say nothing about body composition, and they are not a diagnosis. The finance calculators show the mechanics of a loan or a rate, not an offer, and they do not know your tax position. Each of these pages carries its own disclaimer, and those are the ones to read.

Corrections

If a number here is wrong, the derivation on the page should make it possible to say exactly where it went wrong — which step, which factor. That is the point of showing the working. Report it with the URL and the inputs you used.

Rule Calculator publishes its formulas as an API and an MCP server so an answer taken from here can be checked against the same evaluator that produced it.