How auto loan payments are calculated
An auto loan is a simple fixed-rate installment loan. The lender uses the same amortization formula used for mortgages:
M = P Γ r(1+r)^n / ((1+r)^n β 1)
where P is the amount financed (vehicle price minus down payment and trade-in value), r is the monthly interest rate (APR Γ· 12), and n is the loan term in months. The result is a fixed monthly payment that covers both interest and principal in every period, with the balance reaching zero on the final payment.
Amount financed vs. vehicle price
Your monthly payment is driven by the amount financed, not the vehicle price. Every dollar of down payment or trade-in credit reduces the principal directly, saving you interest on that dollar for every remaining month. On a 60-month loan at 7%, putting an additional $2,000 down reduces total interest paid by roughly $370 and cuts the monthly payment by about $40. This is why negotiating a better trade-in value or saving a larger down payment before buying materially reduces the loan's total cost.
How the APR affects total cost
APR (annual percentage rate) is the annualized cost of borrowing. For a simple installment loan with no fees, APR equals the interest rate. Each month's interest charge is the remaining balance multiplied by the monthly rate (APR Γ· 12). In the first month nearly all of the interest charge applies to the full balance; as the balance falls, so does the monthly interest owed β but the payment stays constant, so more of each payment goes to principal over time. This is standard amortization front-loading.
On a $30,000 loan, the difference between 5% and 9% APR over 60 months is roughly $3,100 in total interest. Getting a lower rate through a credit union, improving your credit score, or making a larger down payment pays dividends across every month of the loan.
Term length and the paymentβcost trade-off
Longer loan terms (72 or 84 months) produce lower monthly payments, but significantly more total interest β and introduce the risk of being "upside down" (owing more than the vehicle is worth) for an extended period. A 72-month loan on a $30,000 car at 7% costs about $1,400 more in interest than a 48-month loan. Shorter terms cost less overall and build equity faster, but require a higher monthly payment.
Most financial advisors suggest limiting auto loan terms to 48β60 months and aiming to keep total car costs (payment + insurance + maintenance) under 15β20% of take-home pay.
Sales tax, registration, and fees
This calculator computes the loan payment on the financed amount only. Sales tax and registration fees vary widely by state and are often rolled into the loan. If you are financing taxes and fees, add them to the vehicle price before entering it here, or reduce your down payment accordingly to reflect the real amount you are borrowing.