Calculation methodology & financial limitations

Monthly installment formula

For a fixed-rate installment loan without a balloon, the scheduled payment is P × r / [1 − (1 + r)⁻ⁿ], where P is principal, r is the nominal annual interest rate (as a percentage) divided by 1,200, and n is the number of monthly payments. At zero interest, payment is P / n. Payments are at month-end. Card payoff, HELOC draw periods and contractual business balloons use their separate rules.

Fees, rounding and extras

Separately paid upfront fees increase borrowing cost but are not financed or deducted from proceeds. Auto has an explicit financed-fees field. The engine uses unrounded amounts internally; displayed and CSV amounts are rounded to cents. Actual lenders may round each installment or accrue interest daily. Extras reduce principal after interest and are capped at the amount owed. No late fees or prepayment penalties are modeled.

Model-specific boundaries

Auto tax deductions are manually entered, not state-specific. Mortgage is conventional fixed-rate only: original value equals purchase price, appraisal is assumed no lower, and modeled PMI automatic termination uses the original scheduled 78% LTV payment date or the month after midpoint, whichever is earlier, assuming current payments. Extra payments do not automatically advance termination; requested 80% cancellation has lender conditions. Housing costs continue after payoff but post-payoff costs are excluded from repayment-period outflows.

Student loans use the entered starting repayment balance and standard fixed payments, without income-driven programs or forgiveness. Cards use constant APR divided by 12, a fixed monthly payment, no new spending or fees and a maximum 600-month payoff. HELOC assumes no new draws, a constant rate, interest-only draw payments and subsequent amortization; extras start in repayment. Business supports monthly installments and an optional principal balloon, not SBA-specific or factor-rate products.

Savings and currency

Savings assumes a constant nominal annual rate divided by 12, monthly compounding and month-end contributions. This rate is not APY. Required contributions are rounded up to cents and the same rounded amount drives the projection. Tax, inflation and account fees are excluded; returns are hypothetical, not guaranteed. Currency changes formatting only. No exchange conversion or non-US lending conventions are applied.

Software verification, not financial certification

The bundled source includes four repeatable Bun test suites covering loan models, loan and savings formulas, difficult numerical boundaries and compare/payoff scenarios. Those suites passed in the 12 October 2026 source check. Testing is not a guarantee of error-free software, external financial approval or a match to every lender contract. Broader device, accessibility and qualified financial review remain advisable.

Using the results responsibly

Verify the entered rate, fees, payment allocation and final obligations with your lender or servicer. Do not use an estimate as an approval decision, current market quote or personalized recommendation. For PMI background, consult the CFPB; for federal student repayment, consult Federal Student Aid.