About NUMOROX

Most financial calculators on the web are wrapped in three advertisements, ask for an email address, and will not tell you what they did. You put in a number, a number comes out, and you have no way to know whether it is right.

That is a poor way to treat someone deciding whether they can afford a house.

NUMOROX takes the opposite position: show the formula, state the assumptions, cite the source when a figure comes from law, and let people check the arithmetic themselves.

Every calculator is a small, separate piece of mathematics with its own automated tests. Expected values in those tests are worked out independently from the textbook formula — never by running the code and recording whatever it said, which proves only that the code has not changed.

That discipline earns its keep. Writing the tests for the mortgage calculator caught a real error: mortgage insurance was being charged at exactly 20% down, because two different thresholds — the 80% at which it is required and the 78% at which it must be cancelled — had been treated as one. Every visitor putting down exactly 20% would have been shown a payment that was too high.

Several worked examples on this site were rewritten before launch because the figures were checked against the code and did not match. The corrected numbers are the ones published.

Tax rates, contribution limits, sales tax rules and lending programme requirements change, and they differ by state and sometimes by county. Rather than build them in — where they would look authoritative and quietly go out of date — the calculators ask you for them, and say where to check.

Where a figure is fixed in law and stable, it is used and the statute is cited on the page.

There are no accounts and no sign-up. The figures you enter never leave your browser, because every calculation runs on your device.

There are no invented reviews, no star ratings nobody gave, no claimed certifications and no fabricated experts. If the site does not say something, it is because we could not honestly say it.

More detail on all of this is in the methodology and the privacy policy.

It is not advice, and it is not a substitute for a professional who knows your situation. Every calculator produces an estimate, and the assumptions behind that estimate are printed beside it so you can judge whether they hold for you.

Corrections, questions and suggestions for what to build next all go to hello@numorox.com, or through the contact page.