Auto Loan APR Calculator and Payment Formula

An auto loan APR calculator estimates your monthly payment and total interest from the amount financed, annual percentage rate (APR), and loan term. Enter those three figures to see an estimate; use the APR and amount from a lender’s disclosure for a loan-specific comparison.

Formula

The calculator treats the loan as a fixed-rate loan with equal monthly payments. It converts the annual APR to a monthly rate, then applies the standard amortizing-loan payment formula:

Monthly payment = P × r ÷ [1 − (1 + r)⁻ⁿ]

Total interest = (monthly payment × n) − P

• P is the amount borrowed, in dollars.
• APR is the annual percentage rate, entered as a percentage such as 8 for 8%.
• r is the monthly rate as a decimal: APR ÷ 1,200.
• n is the number of monthly payments: loan term in years × 12.

The calculator’s payment is rounded to cents in its display. Total interest uses that displayed, rounded payment multiplied by the number of payments, then subtracts the original amount. The last payment on a real loan can differ slightly because lenders calculate and round interest according to their own schedules.

Worked Example

Using the default inputs—$20,000 borrowed, 8% APR, and 5 years:

1. Convert the APR to a monthly decimal rate: 8 ÷ 1,200 = 0.0066667.
2. Convert the term to payments: 5 × 12 = 60 months.
3. Substitute into the payment formula: $20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰] = about $405.53 per month.
4. Calculate total interest using the rounded payment: ($405.53 × 60) − $20,000 = $4,331.80.

These results describe the formula’s estimate for the specified inputs. The exact amount and payment schedule may differ if a lender uses different day-count conventions, payment dates, or rounding.

Assumptions

• The interest rate stays fixed for the full term.
• Payments are equal, made monthly, and occur at the end of each monthly period.
• Interest compounds monthly at APR ÷ 12.
• The calculation excludes fees, taxes, insurance, optional products, and other amounts unless they are included in the amount financed.

What Auto Loan APR Means

APR is an annualized measure of the cost of borrowing, expressed as a percentage. It is not the same thing as the monthly interest rate: for this calculator, an 8% APR becomes a monthly rate of 8% ÷ 12, or about 0.667%. The calculator uses APR as the rate input for the payment formula; it does not independently determine what fees or other costs a particular lender includes in its disclosed APR.

The loan amount is the amount financed, not necessarily the car’s advertised price. A down payment or trade-in value can reduce the amount borrowed. Conversely, amounts added to financing can increase it. To estimate a particular loan, use the principal and APR shown in the lender’s loan documents rather than assuming the vehicle’s price is the loan amount.

How to Read the Results

The monthly payment is the scheduled amount under the formula, not a complete measure of ownership cost. It does not include recurring costs such as fuel, maintenance, insurance, or registration. Total interest estimates the interest paid over all scheduled payments, assuming every payment is made as modeled and the balance reaches zero at the end of the term.

A longer term spreads repayment over more months, which can reduce the calculated monthly payment while increasing the total interest under the same rate and amount. A shorter term does the reverse in this simplified calculation. These are mathematical effects, not a judgment about which loan structure fits a person’s circumstances.

APR and interest rate are sometimes used casually as though they mean exactly the same thing, but a disclosed APR can reflect costs beyond the stated interest rate, depending on the loan and applicable disclosure method. For an accurate comparison, check the lender’s figures for the amount financed, APR, term, payment schedule, and any charges not included in the calculator.

Common Questions About Auto Loan APR

• How do I calculate APR on a car loan?
  The lender determines and discloses the APR using the loan’s rate and relevant costs. This calculator does not derive APR from a payment; it estimates payment and interest when you enter an APR. For the disclosed figure, consult the lender’s documents.
• Is auto loan APR the same as the interest rate?
  Not necessarily. The interest rate is the rate used to calculate interest on the balance. APR is an annualized borrowing-cost measure that may reflect certain costs in addition to the interest rate. Review the loan disclosure to see the figures and included charges.
• Does a lower monthly payment mean less interest?
  No. A payment can be lower because the term is longer, even when the total interest over the loan is higher. Compare the amount financed, APR, term, total of payments, and disclosed costs rather than relying on the monthly payment alone.
• Does this calculator include a down payment or trade-in?
  Not as separate inputs. Enter the resulting amount financed after accounting for any down payment, trade-in credit, or amounts added to the loan. The tool cannot determine those figures for you.
• Why does my lender’s payment differ from this result?
  Possible reasons include fees or add-ons financed into the loan, a different APR or principal, payment dates, daily interest calculations, and rounding. Use the lender’s payment schedule and disclosure for the exact contract figures.

Limits of the Estimate

This is a standard fixed-rate amortization calculation, not a quote or approval decision. It does not model variable rates, skipped or irregular payments, early payoff, late charges, or a final payment adjusted for rounding. It also cannot tell you which APR applies to a borrower or vehicle; obtain actual rates, fees, and loan terms directly from the lender and enter the disclosed figures.

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

Related

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• Final Escrow Statement: What It Shows at Closing
• Escrow Statement Requirements Explained

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