About Rule Calculator
749 pages that answer a question with a number, show the arithmetic that produced it, and name the standard the arithmetic comes from. 64 of them are calculators; the rest are conversions, dates, calendars, charts, guides and printable gauges.
The site exists because the number is usually easy and the checking is not. Anyone can publish that a cubic yard is 27 cubic feet. What is harder to find is a page that shows the multiplication, says which document fixes the figure, and prints the date someone last looked at it.
One specification, six surfaces
Each calculator is written once as a declarative spec — its inputs, its formula, its sources, its worked example. Everything below is generated from that one file, which is why they cannot disagree with each other.
- A page
- the calculator, its derivation, its sources and a worked example
- The chat
- a model picks the calculator and fills its parameters; it never does the arithmetic
- An embed
- the same calculator in an iframe on someone else’s site
- A REST endpoint
- GET /api/calc/{slug} returns the value, every step and the sources
- An MCP tool
- the same calculators, callable by an AI agent over JSON-RPC
- An llms.txt entry
- so an answer engine can find the endpoint rather than scrape the page
What is guaranteed, and what is not
Guaranteed: every figure on the site is produced by a fixed evaluator from inputs you can see, using a formula that is printed on the page. No answer is generated by a language model. Unit factors are the defining ones — an inch is exactly 25.4 mm — rather than rounded table values.
Not guaranteed: prices, which are ranges from published surveys and move with your region and the month; anything describing a body, which is a screening estimate and not a diagnosis; and any figure whose source has changed since it was last checked. Where a number is indicative rather than exact, the page says so in the same place it gives the number.
The formulas and their sources were last reviewed 2026-08-16. How we check describes what the build verifies on every deploy.
What is here
Corrections
A wrong number here is a bug, not a difference of opinion, and it is the one kind of feedback this site most needs. Every calculator page carries a Did this answer your question? control at the foot; answering No is the fastest way to flag one.
There is no published email address yet. Until there is, the control at the foot of each page is the route, and it records which page the report came from.