How We Calculate — Methodology

This page explains the rules every calculator on the site follows. Where a tool has its own assumptions, they're listed on that tool's page.

Formulas and sources

Each calculator uses a standard, published formula. The formula and its variables are shown on the calculator's page. Where a formula depends on a convention, such as how a lender converts an annual rate to a monthly one or how a bank defines APY, we follow primary sources such as US government consumer-finance and investor-education publications and cite them on the page.

Deterministic code, tested

Calculations are ordinary, deterministic code. The same inputs always give the same result. No AI model is involved in calculating any answer.
Every formula has automated tests for typical values, zero, negative numbers, minimums and maximums, decimals, very large values, invalid input and rounding.
Worked examples on each page are produced by running the calculator when the site is built. If an example ever stopped producing a result, the build would fail and the page would not be published.
Each calculator shows a formula version number. It changes whenever the formula or its assumptions change.

Numbers and rounding

Calculations keep full precision internally. Rounding happens only when a result is displayed.
Displayed values are rounded half away from zero (1.005 → 1.01) using decimal-safe rounding, so you won't see artefacts like 0.30000000000000004.
Money is shown to the cent. Lenders round each payment to the cent and adjust the final payment, so their totals can differ from ours by a few cents.
Fraction arithmetic uses exact integer maths, so results are never approximated.

Dates and times

Date calculations use calendar dates only, with no time of day, so results don't shift with your time zone or daylight saving time. Adding months keeps the same day of the month. If that day doesn't exist in the target month, it uses the month's last day (January 31 + 1 month = February 28 or 29). "Today" means today's date on your device.

Validation

Inputs are checked as you type. If a value is missing, out of range or meaningless (for example, dividing by zero), the calculator tells you what to change instead of showing a misleading number.

Limitations

Calculators model the situation described on their page and nothing more. Finance results are estimates: they don't include fees, taxes, insurance (unless entered) or inflation unless the page says so. Always confirm important figures with the provider or a qualified professional.

Reporting an error

If you think a result is wrong, tell us which calculator you used and the inputs you entered. We investigate every report. If we find an error, we fix the formula, add a test that would have caught it, and update the page's version and date.