How formulas are created
Finly begins with standard financial mathematics and primary-source rules. Mortgage tools use fixed-rate monthly amortization. Tax-sensitive tools isolate annual rules in versioned modules rather than scattering values through interface code.
How calculations are tested
Pure TypeScript engines are tested independently from the interface. Tests cover ordinary scenarios, zero rates, bracket boundaries, very small and large values, payoff timing, invalid inputs, and rounding at the display boundary.
Precision and rounding
Calculations retain JavaScript numeric precision through intermediate steps. Currency, percentages, and dates are rounded only when displayed or exported. Final lender or tax-software rounding may differ.
Sources and updates
Mortgage explanations prioritize the Consumer Financial Protection Bureau, tax rules prioritize the Internal Revenue Service, and investing disclosures prioritize the SEC and Investor.gov. Updated and logic-reviewed dates are maintained separately.
Corrections
If a source, assumption, or result appears incorrect, contact hello@finlycalculator.com. Corrections are evaluated against the underlying formula and source before publication.