How to Calculate Your Used Car Payment: Formula and Examples

Understanding the Variables: Loan Amount, APR, and Term

Before you can calculate a monthly payment, you need three key numbers: the loan amount, the APR, and the loan term. The loan amount is the total you borrow after subtracting any down payment or trade-in value from the vehicle price and adding any fees or taxes that you finance. APR stands for Annual Percentage Rate; it reflects the yearly cost of the loan, including interest and certain fees, expressed as a percentage. The term is how long you have to repay the loan, usually stated in months.

Sources: Consumer Financial Protection Bureau

The Standard Auto Loan Payment Formula

The standard formula for calculating the monthly payment on a fixed-rate auto loan is: monthly payment = principal × [rate × (1 + rate)^n] / [(1 + rate)^n − 1]. In this formula, principal is the amount borrowed, rate is the monthly interest rate (APR divided by 12), and n is the number of monthly payments (the loan term in months). This is often called the PMT formula. Because the monthly rate is derived from the APR, the formula assumes monthly compounding.

Step-by-Step Example: $20,000 Loan at 6% for 60 Months

Let’s walk through a complete example.

The number of monthly payments, n, is 60.

Next, calculate the factor (1 + 0.005)^60. This equals approximately 1.3489. The formula for the monthly payment is: payment = principal × [rate × (1 + rate)^n] / [(1 + rate)^n − 1]. The numerator inside the brackets is 0.005 × 1.3489 = 0.0067445. The denominator is 1.3489 − 1 = 0.3489.

How Down Payment and Trade-In Value Change the Equation

A down payment or trade-in reduces the loan amount, which directly lowers your monthly payment. The Consumer Financial Protection Bureau also notes that a smaller loan amount can reduce the risk of owing more than the vehicle is worth.

Sources: Consumer Financial Protection Bureau

Impact of Loan Term on Monthly Payment and Total Interest

The loan term significantly affects both your monthly payment and the total interest you pay. A longer term lowers your monthly payment because you spread the principal over more months, but you end up paying more interest overall. The CFPB provides a clear example using a $20,000 loan at 4.75% APR:

Sources: Consumer Financial Protection Bureau

Using a Spreadsheet to Replicate the Calculation

You can replicate the payment calculation in a spreadsheet program using its built-in PMT function. In Microsoft Excel or Google Sheets, the syntax is =PMT(rate/12, term_months, loan_amount). To see the positive payment, you can wrap the formula in =ABS(PMT(0.06/12, 60, 20000)). This gives you a flexible tool to test different loan amounts, rates, and terms without a specialized online calculator.

If you want to build your own template, open a new spreadsheet and label cells for principal, APR, and term. Then enter the PMT formula in a separate cell, referencing those cells so you can change the inputs and see updated results. You can also calculate the total interest by multiplying the monthly payment by the number of months and subtracting the principal.

Common Mistakes in Calculating Payments

A few common errors can make your calculation inaccurate. One is mixing up APR and the interest rate used for the monthly calculation: always divide the APR by 12 to get the monthly rate. Another is forgetting to subtract your down payment or trade-in from the purchase price before calculating the principal. Also, ensure you use the correct term in months, not years. Finally, remember that the formula gives you the payment for principal and interest only; taxes, fees, and other charges are not included unless you finance them.

The CFPB advises that when comparing loan offers, you should look at more than just the monthly payment. Consider the APR and the total cost over the loan term to fully understand which offer is better for your budget.

Sources: Consumer Financial Protection Bureau