How we calculate
Every calculator on this site follows the same rules, so you can trust its result and see where it comes from.
Standard formulas
Each calculator uses a published, widely used formula. Its page shows the formula, explains each input and gives a worked example, so you can check the result by hand.
Automatic checks
Before every release, more than 3,000 automatic checks compare the calculators with known results, edge cases and invalid input. A change that breaks one of them is not published.
Precision and rounding
Calculations keep full precision; results are rounded only when shown. Numbers, currencies and dates are written the way your language writes them.
Limits
Results are estimates for planning and learning. For financial, medical or legal decisions, check them with a qualified professional.
Corrections
If a result looks wrong, use the feedback form on that calculator's page. Confirmed errors are corrected in the next update.