Auto Loan Payment Calculator Amortization Schedule

An auto loan payment calculator with an amortization schedule estimates your monthly payment and shows how each payment is divided between interest and principal. Enter the amount financed, annual APR, and loan term to see the payment and total interest; the schedule itself can be built by applying the balance formula month by month.

Formula

The calculator uses the fixed-payment loan formula. Let P be the amount financed in dollars, a be the annual percentage rate (APR) as a percentage, and y be the term in years. The monthly interest rate is r = a / 1200, and the number of monthly payments is n = 12y. The unrounded monthly payment is P × r / [1 − (1 + r)⁻ⁿ]. Total interest is estimated as the payment rounded to cents, multiplied by the number of payments, minus the original amount: I = payment × n − P.

The calculator assumes a positive APR, as required by this version of the formula. The payment result is displayed to the nearest cent. Since a real lender may adjust the final payment or use different day-count and rounding conventions, the displayed interest is an estimate based on equal monthly payments.

Worked Example

Using the calculator’s defaults: P = $20,000, a = 8%, and y = 5 years.

1. Convert the annual APR to a monthly rate: r = 8 / 1200 = 0.0066667, or about 0.6667% per month.
2. Find the payment count: n = 5 × 12 = 60.
3. Substitute into the payment formula: $20,000 × 0.0066667 / [1 − (1.0066667)⁻⁶⁰], which is about $405.53 before rounding.
4. Use the rounded payment for total interest: $405.53 × 60 − $20,000 = $4,331.80.

The calculator therefore returns a monthly payment of $405.53 and estimated total interest of $4,331.80. Multiplying the rounded payment by 60 gives total scheduled payments of $24,331.80, of which $20,000 repays principal and $4,331.80 is interest in this model.

Assumptions

• The interest rate stays fixed for the entire loan term.
• Payments are equal, made monthly, and applied at the end of each monthly period.
• Interest compounds monthly using APR divided by 12.
• The calculation excludes fees, taxes, insurance, optional products, and any down payment or trade-in unless those amounts are already reflected in the amount financed.

How an Amortization Schedule Works

An amortization schedule is a table of the loan’s expected payments and remaining balance. For each month, interest is calculated on the balance at the start of that period. The payment first covers that interest; the rest reduces principal. The following month’s interest is calculated on the lower balance.

For a month-by-month schedule, use interest_k = balance_k−1 × r, then principal_k = payment − interest_k, and balance_k = balance_k−1 − principal_k. Start with the original principal as the balance before the first payment. In the example, first-month interest is about $133.33, so about $272.20 of the $405.53 payment reduces principal. Later payments generally contain less interest and more principal, provided the rate and payment remain unchanged.

After k payments, the theoretical balance can also be calculated as P(1 + r)^k − payment × [(1 + r)^k − 1] / r. A schedule rounds each transaction to cents, so its displayed balance can differ slightly from this formula, which uses unrounded intermediate values. The final payment may consequently differ from the regular payment.

What to Enter and How to Read the Results

• Loan amount: Enter the amount actually financed, not necessarily the vehicle’s advertised price. If a down payment, trade-in credit, or other amount changes the principal, reflect that in the figure you enter.
• APR: Use the annual rate supplied for the specific loan offer. Do not enter a monthly rate in this field; the formula converts APR to a monthly rate by dividing by 12 and by 100.
• Term: Enter the repayment period in years. The calculator converts years to monthly payments, so a five-year term means 60 scheduled payments.
• Monthly payment: This is the modeled regular payment for principal and interest, not necessarily the complete amount due on a bill.
• Total interest: This is the modeled interest over the full scheduled term, using the rounded monthly payment. It does not include other borrowing or vehicle ownership costs.

To create a full schedule, repeat the monthly interest and principal steps for each payment and record the remaining balance. A calculator that displays only payment and total interest does not itself provide every monthly row; the formulas above explain how those rows are derived.

Costs, Comparisons, and Common Calculation Errors

The amount financed is not always the vehicle’s purchase price. Taxes, registration charges, dealer or lender fees, optional products, a down payment, and trade-in value can change the principal or the amount paid separately. This calculator has no separate fields for those items, so use the lender’s stated amount financed if you want an estimate tied to a particular offer. For the real APR, fees, and payment terms, consult the lender’s disclosures rather than relying on an illustrative input.

When comparing two hypothetical loan structures, keep the amount financed and rate assumptions clear. A longer term can lower the scheduled payment while increasing the number of months over which interest accrues; a lower payment by itself does not reveal total borrowing cost. This is arithmetic, not a recommendation about which loan to choose.

• Entering 8 instead of 0.08 is correct for an APR field expressed as a percent; the formula divides by 1200.
• Using the vehicle price instead of the financed principal can overstate or understate the estimate.
• Rounding the monthly payment before calculating total interest matters. Here, the total uses the displayed payment rounded to cents.
• Assuming the output includes insurance, taxes, or fees can make a monthly budget estimate incomplete.
• Assuming every lender calculates interest and adjusts the last payment identically can produce a mismatch with an actual statement.

Frequently Asked Questions

What does an auto loan amortization schedule show?
It shows each scheduled payment, the interest and principal portions, and the remaining loan balance. Early payments generally allocate a larger share to interest than later payments under a fixed-rate, level-payment model.

How do I calculate the interest portion of a car payment?
Multiply the balance at the start of the month by the monthly rate, calculated here as APR divided by 1,200. Subtract that interest from the payment to estimate the principal reduction.

Why is the lender’s payment different from this calculator?
The amount financed, APR, term, payment dates, fees, add-ons, and rounding may differ from the inputs or assumptions. Check the loan disclosure and payment schedule for the figures used on the actual contract.

Does the monthly payment include insurance and taxes?
No. This calculation estimates principal and interest only. Insurance, taxes, and other charges are not included unless they have been incorporated into the amount financed, and this calculator cannot identify them separately.

Does paying extra change the amortization schedule?
Yes, if an extra amount is applied to principal, the balance and future interest can change. This calculator assumes only the fixed regular payment and does not model extra payments, payment timing changes, or a revised payoff date.

This result is an estimate, and the lender’s disclosure gives the exact figures.

Related

• Auto Loan Calculator With Trade-In and Amount Owed (Negative Equity)
• Car Loan Calculator With Down Payment: Monthly Car Payment

in p | Loan amount | $ | 20000 | 1 | 100000000
in apr | Interest rate (APR) | % | 8 | 0.01 | 100
in yrs | Loan term | years | 5 | 0.0833 | 50
out pmt | Monthly payment | $ | p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2