Methodology
How these calculators are built, tested and kept correct — and what they deliberately do not do.
How we build a calculator
Six rules that every calculator on this site follows.
The formula is on the page
Every calculator states the equation it uses, in the notation you would find in a textbook, along with what each term means. If you cannot check our arithmetic, you have no reason to trust it.
Assumptions are named, not buried
Most financial questions only have an answer once you fix some assumptions — a constant rate of return, a property tax that never rises, a fifty-two week year. We state each one where it affects the result, because those assumptions are usually where two calculators disagree.
Impossible values are refused
A negative price or a 200% interest rate produces a message beside the field, not a confident wrong answer. Silently coercing bad input into a plausible-looking number is the worst thing a calculator can do.
Figures that change are sourced and dated
Tax brackets, contribution limits, wage bases and lending rules change every year. Where a calculator depends on one, the value is stored with its source and the date we last checked it, and shown on the page. We do not work from memory on anything with a publication date.
Every formula is unit tested
Each calculation module has a test suite with known inputs and known results, computed independently rather than read back out of our own code. The suites cover zero, negative, empty and extreme values, decimal inputs, rounding, and the invariants that must always hold — an amortization schedule that sums to the amount borrowed, a breakdown that sums to its total.
Rounding follows the money
Intermediate values are rounded to the cent the way a servicer or a payroll system rounds them, rather than carried at full floating-point precision and rounded once at the end. It is why our schedules match a real statement, and why a final payment absorbs a few cents of drift.
What happens to what you type
Nothing.
Calculations run in your browser. The numbers you enter are held in the page while you are on it and are gone when you close the tab. They are not sent to a server, not written to your device, and not recoverable by us — there is no account to attach them to and no database to put them in.
That is a consequence of how the site is built rather than a promise about our intentions, which is the only kind of privacy claim worth making. It also means we cannot restore your inputs if you navigate away.
What these calculators are not
The honest limits, in the same place as the claims.
They are estimates for planning, not offers, quotes, or advice. A lender, an employer or an accountant works from facts we do not have — your credit, the specific property, your filing situation, your state — and their number is the one that counts.
They also do not know what you should do. A calculator can tell you that a fifteen-year mortgage costs less in total and more each month. Whether that trade is right for you depends on things a number cannot see.
If you find a result you believe is wrong, we would rather hear about it than not. Tell us the inputs you used and the figure you expected, and we will check the formula and the tests against it.
NUMOROX provides informational calculators and estimates. It is not financial, tax or legal advice. Back to the calculators.