How Auto Loan Payments Are Calculated
Auto loans use the same fixed-rate amortization formula as mortgages: M = P[r(1+r)^n] / [(1+r)^n - 1], where P is the amount financed (vehicle price minus down payment and trade-in), r is the monthly interest rate, and n is the total number of payments. Auto loans are typically short-term: 24, 36, 48, 60, or 72 months.
How Much Car Can You Afford?
A widely used guideline is the 20/4/10 rule: put at least 20% down, finance for no more than 4 years, and keep total vehicle costs (loan payment plus insurance) at or below 10% of your gross monthly income. Longer loan terms lower the monthly payment but significantly increase total interest and the risk of being "upside down" on the loan.
Total Cost of the Loan
On a $35,000 loan at 7% for 60 months, the monthly payment is approximately $693 and total interest is approximately $6,583. Extending the same loan to 72 months reduces the monthly payment to approximately $597 but increases total interest to approximately $7,963.
Frequently Asked Questions
Should I put more money down? A larger down payment reduces the amount financed, lowering both your monthly payment and total interest, and reduces the risk of negative equity in the first two years.
Can I pay off my auto loan early? Most auto loans allow early payoff without penalty. Even modest extra payments each month toward principal can significantly reduce total interest and shorten the loan term.
New vs. Used Car Loan Rates
Lenders typically charge higher rates for used vehicles than new ones, reflecting greater risk and faster depreciation. A borrower who qualifies for 6% on a new car might see 8% to 9% on a used car of the same loan amount. Factor this spread into a new-vs-used decision alongside the purchase price difference.
Common Auto Loan Mistakes
Negotiating the monthly payment instead of the price. Dealers can hit a target monthly payment by extending the term rather than lowering the price -- always negotiate purchase price and financing separately, then check the total interest cost.
Skipping a pre-approval before visiting the dealership. A bank or credit union pre-approval gives you a baseline rate to compare against dealer financing, which often includes a markup over the lender's actual buy rate.
Frequently Asked Questions
Is a longer loan term ever a good idea? It can lower your monthly payment, but it also extends the period of negative equity and increases total interest -- only worthwhile if the shorter-term payment genuinely does not fit your budget.
Does refinancing an auto loan make sense? If your credit has improved or rates have dropped since your original loan, refinancing can lower your rate -- but confirm there's no prepayment penalty on your current loan first.
The Inputs, Field by Field
Vehicle price. The out-the-door price you have negotiated. Negotiate this number before you ever discuss a monthly payment, because a dealer can hit any monthly target simply by stretching the term.
Down payment and trade-in. Cash down plus any trade-in equity both reduce the amount financed. Together they are your best lever against negative equity in the first two years, when a new car depreciates fastest.
Interest rate (APR). The annual rate your lender or the dealer's financing offers. A pre-approval from your own bank or credit union gives you a baseline to compare against dealer financing, which often carries a markup over the lender's actual buy rate.
Loan term. The number of months to repay -- typically 24 to 72. Longer terms lower the monthly payment but raise total interest and lengthen the window in which you owe more than the car is worth.
How to Read Your Results
The monthly payment is what you will owe each month for principal and interest. The total interest figure is the true cost of financing: compare it across term lengths and you will often find that 72 months instead of 60 adds hundreds or more in interest for a modest drop in the monthly payment. Watch the early balance too -- if the amount you owe stays above the car's resale value for a long stretch, you are "upside down," which matters if the vehicle is totaled or you want to sell before payoff.
Assumptions and Limitations
This calculator assumes a fixed APR and equal monthly payments. It does not include sales tax, title, registration, documentation fees, or optional add-ons such as an extended warranty or gap insurance unless you fold those into the amount financed. It also excludes insurance premiums, which the 20/4/10 rule counts toward your total vehicle cost. Results are educational estimates for comparison, not a financing offer.
Related Calculators and Guides
Before you shop, size up your budget with our how much car can I afford guide and the guide to financing a car. If you already have a loan, the refinance calculator shows whether a better rate is worth it, and the loan comparison calculator puts two offers side by side.