Auto Loan Interest: Calculate Payments and Total Cost

Auto loan interest rates are the annual borrowing costs expressed as an APR, but your actual rate depends on the lender’s offer and your application; it cannot be determined from the loan amount alone. Use this calculator to estimate the monthly payment and total interest by entering the amount financed, APR, and term, then replace the example APR with the rate shown in a lender’s disclosure.

Formula

The calculator assumes a fixed APR and equal monthly payments. It converts APR to a monthly rate, calculates a level payment, rounds that payment to the nearest cent, and multiplies it by the number of monthly payments to estimate total interest.

• Monthly rate: r = APR / 1200, where APR is the annual percentage rate entered as a percentage and r is the monthly decimal rate.
• Number of payments: n = years × 12, where years is the loan term in years and n is the number of months.
• Monthly payment: payment = round(P × r / [1 − (1 + r)⁻ⁿ], 2), where P is the amount financed in dollars.
• Total interest: interest = payment × n − P, with both payment and P in dollars.

For example, the APR input of 8 means 8% per year; dividing by 1,200 converts it to a monthly rate of 0.0066667. The formula presumes that interest accrues monthly at APR / 12 and that each scheduled payment is made on time. The stated APR is an input, not a claim about rates currently available.

Worked Example

Using the calculator’s defaults—$20,000 financed, 8% APR, and a five-year term—the calculation proceeds as follows:

1. Convert the APR to a monthly rate: 8 / 1200 = 0.0066667.
2. Find the number of payments: 5 × 12 = 60.
3. Apply the payment formula: $20,000 × 0.0066667 / [1 − (1.0066667)⁻⁶⁰] = $405.53 per month, rounded to cents.
4. Multiply the rounded payment by the number of payments: $405.53 × 60 = $24,331.80 in scheduled payments.
5. Subtract the amount financed: $24,331.80 − $20,000 = $4,331.80 total interest.

The estimate includes only the principal and interest represented by the inputs. The actual final payment or total can differ if the contract uses different payment dates, rounding, or other charges.

Assumptions

• The interest rate is fixed for the full term.
• Payments are made monthly, at the end of each monthly period, and on schedule.
• Interest compounds monthly using APR divided by 12.
• Fees, taxes, insurance, optional products, and other charges are not included unless they are part of the amount financed entered as P.

What Affects an Auto Loan Rate?

An auto loan APR is set by the lender and may vary with details of the application and the transaction. The figure to use in this calculator is the APR offered for the specific loan, rather than a general advertised figure or an estimate from a different borrower’s situation. Ask the lender for the APR, amount financed, term, payment schedule, and any charges that affect the total amount due.

Factors that can be considered in an individual offer include credit and income information, the vehicle and its age, the amount borrowed, the length of the term, and whether the vehicle is new or used. Their effect is lender-specific; this calculator cannot predict approval, determine an offered rate, or tell you how a particular application will be assessed. A co-borrower, down payment, or trade-in may change the transaction’s amount or terms, but the lender must provide the actual figures.

APR, Interest Rate, and Total Cost

The interest rate describes the rate used to calculate interest on the outstanding balance. APR is a standardized annualized measure intended to reflect the borrowing cost, including certain finance charges where applicable. How charges are treated in a particular offer can depend on the contract and applicable rules, so check the lender’s disclosure rather than assuming every fee is included in the calculator’s APR.

Monthly payment alone does not show the full cost. For example, a longer term generally spreads repayment over more months and can lower the calculated monthly payment at a given principal and APR, while increasing the number of periods during which interest accrues. Compare written figures using the same amount financed and the same assumptions. If fees are financed, include them in the amount financed only when that matches the lender’s disclosed figure; do not add them again as separate interest.

Using the Estimate and Avoiding Common Errors

Enter the amount actually financed, not necessarily the vehicle’s sticker price. A down payment or trade-in can reduce the amount borrowed, while taxes, fees, or other items financed into the contract can increase it. Then enter the APR as a percentage—8 for 8%, not 0.08—and enter the term in years. The calculator uses 12 payments per year.

• Use the APR from the specific written offer, not a monthly rate or a rate from an unrelated example.
• Check whether the lender’s amount financed includes charges or optional products.
• Do not treat the result as a payoff quote. A payoff amount can depend on the date, accrued interest, and contract terms.
• Remember that displayed payments and totals are rounded; a lender’s schedule may have a slightly different final payment.

For a more detailed comparison, obtain the payment schedule and total of payments for each offer, along with the amount financed and APR. This keeps the comparison grounded in the contract figures rather than estimates assembled from different assumptions.

Limits of the Calculation

This is a standard fixed-rate amortization estimate, not a loan offer or an approval assessment. It does not model variable rates, irregular payment timing, late or missed payments, extra principal payments, deferred payments, balloon balances, or changing charges. It also cannot determine taxes, insurance, registration costs, or the market value of a vehicle. If a contract’s payment structure differs from equal monthly payments, use its disclosed schedule instead.

Frequently Asked Questions

How much interest will I pay on an auto loan?
The amount depends on the financed principal, APR, term, payment timing, and contract details. Enter the actual amount financed, APR, and term above for a fixed-rate estimate; the lender’s disclosure provides the contract figures.

Is auto loan APR the same as the interest rate?
Not necessarily. The interest rate is used to accrue interest on the balance, while APR is an annualized borrowing-cost measure that may account for certain finance charges. Check the offer disclosure for what is included.

Does a longer auto loan term reduce interest?
Not necessarily. At the same principal and APR, a longer term usually lowers each scheduled payment but extends repayment, which can increase total interest in this fixed-rate model.

Does this calculator include a down payment?
It does not have a separate down-payment field. Enter the amount financed after the down payment and any trade-in credit, adjusted for charges included in the loan, if that matches the lender’s figure.

Why is my lender’s payment different?
The offer may use a different amount financed, APR, term, payment date convention, fees, or rounding method. Compare the inputs and the lender’s payment schedule; this estimate models equal monthly payments and rounds the payment to cents.

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

Related

• Loan Calculator Monthly Breakdown: Payment and Interest
• Advanced Loan Payment Calculator: Payments and Interest
• Calculate a Monthly Loan Payment: Formula and Example

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 | $ | round(p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2