Methodology
This page describes how the calculators on CalcMyWay are built and verified. It exists so that you can judge whether to trust a result, rather than having to take it on faith.
Where formulas come from
Calculators implement standard, published formulas — the amortising-loan payment formula, the future value of an annuity, the WHO body mass index classification, SI unit definitions. We do not invent proprietary models. Where a formula has more than one accepted form, the calculator states which one it uses and the page explains the difference.
Conventions we had to choose
Many calculations have no single correct convention, only a common one. In those cases we pick the convention most users expect, state it plainly on the page, and where practical offer a setting to change it. Examples:
- Interest rates. Periodic rates are derived from the nominal annual rate divided by the number of periods, which is the convention used by US lenders, rather than by geometric conversion from an effective rate.
- Tipping. Tips are calculated on the pre-tax subtotal when a tax rate is supplied, which is the traditional US convention.
- Date counting. Date differences count one end by default; an option includes both, which is what you want for things like booked leave.
Precision and rounding
Intermediate calculations are carried at high precision and rounded only for display. Running balances in amortisation schedules use arbitrary-precision decimal arithmetic rather than floating point, because floating-point error accumulates visibly over 360 monthly periods and would make the final balance fail to reach zero. Displayed money values are rounded half away from zero, which is what financial convention expects.
Testing
The shared financial, date and formatting functions are covered by unit tests that check them against known-correct reference values, including the edge cases that break naive implementations: zero interest rates, payments too small to amortise, leap years, and payments that clear the balance early.
Review and dating
Each calculator page shows the date it was last substantively reviewed. That date changes only when the calculation, the assumptions or the explanation changed — not when the page was merely rebuilt. Where a calculator encodes figures that go out of date (tax rates, statutory thresholds), the figure is shown with its source so you can check it against the current rule.
Reporting an error
Please report anything that looks wrong via the contact page. Include the calculator, your inputs, the result you got and the result you expected.