How we verify our math

Calculator sites live or die on trust. Here is exactly how this one works.

Deterministic code does the arithmetic

Every calculator is a small, pure function with typed inputs and outputs. Each one ships with automated tests that check the defaults, the worked examples, edge cases like zeros and extremes, unit handling, and — where a textbook value exists — the answer itself (a $320,000 loan at 6.5% for 30 years must come out to $2,022.62). The test suite runs before every deployment; if a calculator fails, the site doesn’t ship.

What the AI does

• Routing: it reads your question, picks the calculator, and extracts the numbers you typed. The numbers are then validated against the calculator’s rules.
• Explaining: it reads the exact results on the page and describes them. For any what-if it re-runs the calculator through a tool call; it is instructed never to compute a number itself.
• Building: when you ask for a calculator we don’t have, it writes the formulas as a declarative spec. Our engine checks every expression parses and evaluates to finite numbers before it’s saved. Generated calculators are labelled and not indexed until reviewed.

Show your work

Every calculator exposes its formula and a step-by-step breakdown with your numbers substituted in. If you disagree with a result, you can see exactly where.

Sources and updates

Any calculator that depends on real-world constants (tax brackets, health thresholds, physical constants) cites its sources and shows when they were last checked. Head-term calculators pull live defaults where a reliable public series exists — the mortgage calculator opens with this week’s Freddie Mac average rate.

No ads, no accounts

There is nothing on these pages except the calculator and its explanation. Your inputs live in the URL, so sharing or saving a calculation is just copying a link. Nothing is stored server-side unless you build a calculator, in which case the spec is saved so the link works for everyone.

Found a mistake? The API makes it easy to reproduce, and every page shows its formula — please tell us.