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Calculation methodology and sources

What the calculators do, what they assume, and where an estimate stops. No current rate or approval claim is built into the math.

Fixed-rate payment formula

For a standard fully amortizing loan, the scheduled monthly payment is calculated from the amount financed (P), monthly interest rate (r), and number of payments (n).

Payment = P × [r(1 + r)ⁿ] ÷ [(1 + r)ⁿ − 1]
  • At 0% interest, the amount financed is divided evenly by the number of payments.
  • Calculations retain full precision; displayed currency is rounded to two decimal places.
  • The final payment is reduced when the remaining principal is smaller than the scheduled principal payment.

Vehicle amount financed

Out-the-door cost

Vehicle price minus cash rebate, plus estimated tax and every fee or add-on entered.

Trade equity

Trade allowance minus the lender payoff amount. Positive equity reduces borrowing; negative equity increases it.

Amount financed

Out-the-door cost minus cash down and trade equity, never below zero.

Tax handling

A manual estimate. The user chooses whether a rebate or trade allowance reduces the taxable price because local rules vary.

APR, rate, and payoff limitations

  • The payment engine uses the annual interest-rate input divided by 12. A disclosed APR can include certain fees, so entering APR is an approximation unless the contract uses that rate for periodic interest.
  • A lender payoff amount can differ from a statement balance because it can include interest through a particular date and unpaid charges.
  • Taxes, incentives, trade credits, payment timing, compounding, fees, insurance, and prepayment rules vary. Replace every illustrative default with the actual written quote.
  • Biweekly figures are shown only as a same-annual-payment cash-flow equivalent unless the calculator explicitly models a 26-payment schedule.

Sources and provenance

Methodology checked on 18 July 2026 against the public consumer guidance below. PayoffCalculator.io is the publisher; no credentialed financial reviewer is claimed.

Version

2026-07-18

These calculators provide educational estimates, not financial, tax, or legal advice. They do not determine approval, eligibility, or an available rate, and PayoffCalculator.io is not a lender.