Auto Loan Payment Calculator with Extra Payments
This calculator estimates auto-loan payoff timing and interest when you make extra payments. It supports recurring extras applied each payment, biweekly payment schedules, and one-time lump-sum prepayments.
Use the tool to compare outcomes: how much sooner you can pay off the loan and how much interest you may save. Results are estimates based on standard amortization math and assume no additional fees, penalties, or changes to your APR.
Standard amortizing monthly schedule plus a fixed extra payment applied each payment period.
Inputs
Results
Scheduled monthly payment (no extras)
$377.42
Monthly payment including recurring extra
$377.42
Estimated payoff time (months)
—
Total interest paid (with extras)
—
Interest saved (vs original schedule)
—
| Output | Value | Unit |
|---|---|---|
| Scheduled monthly payment (no extras) | $377.42 | USD |
| Monthly payment including recurring extra | $377.42 | USD |
| Estimated payoff time (months) | — | months |
| Total interest paid (with extras) | — | USD |
| Interest saved (vs original schedule) | — | USD |
Visualization
Methodology
Calculations use standard amortization formulas for fixed-rate loans: periodic interest rate = APR / periods per year. The scheduled payment formula solves for the level payment that amortizes the loan over the term.
Early payoff with extra payments is estimated using closed-form algebraic rearrangements of the amortization recurrence (logarithmic solution for remaining periods) and standard remaining-balance formulas to handle lump-sum prepayments.
Modeling and implementation follow recognized engineering and information-security practices; pay attention to input ranges, rounding, and the calculator's stated limitations as recommended by NIST, ISO, and IEEE standards for numerical software.
Further resources
Expert Q&A
How accurate are the results?
Results are numerical estimates using standard amortization formulas. They exclude taxes, fees, prepayment penalties, changing interest rates, or any lender-specific rounding rules. Small rounding differences may occur versus a lender's exact schedule.
What if the APR is zero?
The formulas assume a positive APR. For a 0% APR loan, repayment time simplifies to principal divided by payment. Enter a small positive APR or interpret outputs accordingly; the tool will not model special-case lender rounding for exact zero-rate edge cases.
Does this account for prepayment penalties or fees?
No. The calculator does not include lender-specific prepayment penalties or administrative fees. If your loan has penalties, adjust inputs or consult your lender and a financial advisor.
Why do biweekly schedules save interest?
Biweekly schedules typically result in more frequent payments and effectively an extra partial payment per year, reducing principal faster and lowering total interest. The model computes an equivalent periodic schedule to estimate this effect.
Can results be used for regulatory disclosures?
This tool provides educational estimates and is not a substitute for formal regulatory disclosures or lender statements. For legally binding payoff figures, obtain an official payoff quote from the lender.
Sources & citations
- NIST — Numerical Methods and Software Engineering Guidance — https://www.nist.gov
- ISO — Standards for software quality and numerical accuracy — https://www.iso.org
- IEEE — Best practices for algorithmic implementation and testing — https://standards.ieee.org
- OSHA — General workplace safety and documentation practices — https://www.osha.gov