Auto Loan Amortization Calculator with Extra Payments

The calculator uses standard amortization mathematics: a periodic interest rate r = APR / payments_per_year; scheduled payment computed by the annuity formula. For recurring extra payments it solves for the number of payments required with a higher periodic payment using the analytic payoff formula.

For one-time lump sums the outstanding balance before the extra is computed using the closed-form remaining-balance expression, the lump sum is subtracted, and the remaining payoff is recalculated. Biweekly results use a 26-payments-per-year model as an approximation; some lenders instead recompute monthly and apply splits differently.

Numerical stability and rounding: computations round monetary outputs to typical currency precision when displayed, but internal calculations use higher precision. Users should treat results as estimates; consult your loan contract or lender for exact posting and prepayment policies.

Use the recurring extra mode to see how consistent extra contributions accelerate payoff and reduce interest.

Use the one-time extra mode to evaluate lump-sum prepayments at a particular payment number.

Biweekly equivalent is an approximation; actual lender processing and rounding rules may create small differences.

Example 1: $25,000 loan, 5.0% APR, 5 years (monthly). Scheduled payment ≈ computed by the annuity formula. Adding $50 per month reduces the number of payments and saves interest; the calculator returns estimated payments saved and interest saved.

Example 2: Same loan, apply a $1,000 one-time payment at payment number 12. The tool computes the balance just before that payment, subtracts $1,000, recalculates remaining payments and total interest, and reports savings versus the standard plan.