Auto Loan Interest Calculator with Extra Payments
This calculator estimates payments, payoff time, and interest for auto loans and shows how one-time or recurring extra payments change the outcome. Use it to compare a baseline monthly amortization against accelerated options.
Enter your loan amount, APR and term. Add a one-time extra or recurring extra payment to see updated payoff months and estimated interest savings. For biweekly schedules the tool uses 26 payments per year (two payments per month in most months) to illustrate typical acceleration effects.
Baseline monthly amortization schedule without extra payments used as the comparison point for savings.
Inputs
Advanced inputs
One-time extra payment
Recurring extra payment
Results
Monthly payment
-$1.53
Term (months)
60
Total interest (no extras)
-$20,091.67
| Output | Value | Unit |
|---|---|---|
| Monthly payment | -$1.53 | USD |
| Term (months) | 60 | months |
| Total interest (no extras) | -$20,091.67 | USD |
Visualization
Methodology
Monthly amortization uses the standard fixed-rate loan formula: payment = P * r / (1 - (1+r)^-N) where r is the periodic rate and N is the number of periods.
One-time and recurring extra payment effects are computed using closed-form formulas for remaining balance and the analytical solution for number of periods remaining using logarithms when the periodic rate is non-zero.
Biweekly schedules use a 26-payment-per-year convention; where exact day-level accrual matters (e.g., leap years or differing billing cycles), results are approximate. All calculations assume payments are applied at scheduled intervals and interest compounds at the stated periodic rate.
Key takeaways
Use the standard amortization results as the baseline. Adding extra payments (one-time or recurring) reduces remaining balance and total interest; recurring small amounts often compound into meaningful savings.
Treat all outputs as estimates. Confirm exact payoff and savings with your lender because billing practices, daily accrual, rounding and fees can change results.
Further resources
External guidance
Expert Q&A
Does the calculator account for fees, taxes or insurance?
No. This tool models interest-only amortization on the principal and does not include separate fees, title taxes, GAP insurance, or other charges. Add those to the principal if you want them included in the schedule.
How accurate are the savings estimates for extra payments?
Estimates use analytical formulas and assume scheduled application and compounding at the stated periodic rate. If your lender applies payments differently (daily interest accrual, minimum payment rules, or prepayment penalties) your actual savings may differ.
What happens if APR is zero?
Formulas that divide by the periodic rate require special handling when APR = 0. The calculator falls back to equal principal division (principal / number of periods) in that case. If you rely on a zero-APR promotion, verify with lender statements.
Are biweekly results exact?
Biweekly estimates assume 26 equal payments per year and that each payment is applied immediately. Exact amortization can differ if your lender reindexes payment dates, charges fees, or applies daily interest. Use the biweekly mode as an illustrative comparison.
Sources & citations
- NIST - National Institute of Standards and Technology — https://www.nist.gov
- ISO - International Organization for Standardization — https://www.iso.org
- IEEE - Institute of Electrical and Electronics Engineers (IEEE 754 floating-point guidance) — https://www.ieee.org
- OSHA - Occupational Safety and Health Administration — https://www.osha.gov
- Consumer Financial Protection Bureau - Loan disclosures and APR — https://www.consumerfinance.gov