Last updated: September 26, 2026
How our calculators work
Every PayoffCalc calculator runs entirely in your browser using JavaScript. Your inputs are never sent to a server (see Privacy Policy). Each tool implements standard, textbook financial formulas — the same math a bank or spreadsheet uses — and shows you the breakdown, not just the headline number.
Verification
Calculator logic is unit-tested against independently computed values before release:
- Credit card payoff: month-by-month amortization simulation cross-checked against the standard payoff formula; impossible-payment detection (when interest exceeds the payment) verified.
- Loan amortization: payment formula `M = P·r(1+r)ⁿ/((1+r)ⁿ−1)` verified against published amortization tables; extra-payment and biweekly schedules validated by simulation.
- Reverse sales tax: `pre-tax = total ÷ (1 + rate)` verified against forward-tax computation.
- Projections (457(b), VOO, compound interest): future-value-of-annuity math verified against closed-form formulas.
- Dividend estimators: `annual income = investment × yield` — simple by design; the key input (yield) is user-assumed and labeled as variable, never presented as guaranteed.
Edge cases are tested explicitly: zero interest rates, zero balances, payments too small to cover interest, and empty inputs.
Assumptions we disclose
Each calculator page documents its assumptions. Common ones:
- Interest compounds monthly unless stated otherwise.
- Projections assume a constant rate of return — real markets vary; see the Financial Disclaimer.
- Tax treatment is simplified; consult a tax professional for your situation.
- Fees, inflation, and plan-specific rules (vesting, loan provisions) are noted where relevant but not modeled unless the tool says so.
What calculators can’t do
A calculator computes; it doesn’t advise. It doesn’t know your emergency fund, your job stability, your tax bracket, or your goals. Use outputs as a starting point for decisions — not the decision itself.
Reporting a math error
If you believe a calculator is wrong, tell us via the Contact page with your inputs and expected result. Math errors are treated as top-priority fixes.