How we calculate results
Documented, tested calculations
Every calculator uses pure calculation functions: with the same inputs and formula version, the result is always the same. Automated tests compare results with independently calculated reference values, including edge cases such as zero interest, zero duration or very large amounts.
Decimal arithmetic and rounding
Amounts are calculated with decimal arithmetic rather than binary floating point, to avoid cent-level errors. We only round at the end, or when a real amount is fixed: for example, a loan payment is rounded to the cent and the final payment absorbs the difference. Rounding is commercial (half up) and follows each currency's decimals; the Chilean peso, for example, has none.
Explicit assumptions
Every page states its assumptions: compounding frequency, contribution timing (start or end of period), constant interest rates and which costs are excluded, such as taxes, fees or insurance. Each calculator's worked example is illustrative, not a guaranteed outcome.
Country tax rules
Taxes, contributions and pensions vary by country and year. Official rules come from official sources, are stored by country and tax year, and are reviewed before being built in as defaults. Until then, those calculators use the percentages, brackets or ages you enter, and each page says so in its assumptions.
Versions and errors
Every calculator shows its formula version. If you believe a result or rule is wrong, tell us the calculator, country, tax year and the values you entered through the contact page.