Methodology
Shared modeling conventions used across the calculator catalog.
Marlecourt treats calculation engines as production software: documented, tested, and separated from presentation. Each calculator page explains the method, assumptions, and limitations for that tool. This page summarizes site-wide conventions.
Rates: APR vs APY vs effective return
Loan tools generally use a nominal APR with monthly interest conventions. Savings and many investing accumulation tools use an effective annual return (APY-style) unless a calculator explicitly asks for compounding frequency. APR and APY are not interchangeable — use the APY vs APR calculator when converting between them.
Contributions and accumulation
Recurring contributions are typically modeled at the end of each contribution period unless a calculator offers beginning-of-period timing. Horizons are capped where needed so models stay finite and responsive.
Loans and amortization
Standard amortizing loans use monthly interest on the remaining balance. Where schedules are shown, payment allocation is tracked in cents-compatible money math so principal and interest reconcile to the modeled balance path.
Inflation and today's dollars
Long-term tools that report today's dollars apply a user-supplied inflation assumption. Nominal future dollars and inflation-adjusted dollars are labeled separately when both appear.
Taxes
After-tax and capital-gains tools use only the estimated tax rate (and simplified cost basis, where applicable) that you supply. The site does not apply hard-coded federal or state brackets, filing-status logic, holding-period classification, NIIT, wash-sale, or loss tax-benefit engines.
Rounding
Internal math keeps available precision. Currency is typically rounded to two decimal places at display time. Percentages usually use two decimals unless extra precision helps interpretation.
Model limitations
Launch calculators prioritize evergreen mathematical relationships over live market rates, jurisdiction-specific programs, or product quoting. A result can also be a modeled limitation (for example, a goal not reached within the horizon) rather than invalid input.
For calculator-specific formulas, examples, and FAQs, open the methodology section on each calculator page, or start from the calculator directory.