How Auto Loans Work
An auto loan is a fixed-rate, fixed-term installment loan secured by the vehicle itself. Unlike a credit card or personal line of credit, you borrow a set amount and repay it in equal monthly installments over a fixed period — typically 36 to 84 months. The lender holds a lien on the vehicle title until the loan is paid off, at which point the title transfers to you fully.
Your monthly payment is determined by four variables: the loan amount (principal), the interest rate (APR), and the loan term. The loan amount is the vehicle price plus any sales tax financed, minus your down payment and any net trade-in equity. This calculator shows you exactly how those pieces combine into your monthly obligation.
How the Monthly Payment Is Calculated
Auto loan payments use the standard amortization formula — the same math behind personal loans, mortgages, and any other fixed-rate installment debt:
This formula produces the fixed monthly payment that will exactly pay off the loan over the term, with each payment covering the interest accrued since the last payment plus a reduction in the principal balance. The loan amortizes to exactly zero at the final payment.
Monthly rate r = 6% ÷ 12 ÷ 100 = 0.005
(1 + r)^n = (1.005)^60 = 1.34885
M = $30,000 × (0.005 × 1.34885) / (1.34885 − 1)
M = $30,000 × 0.006744 / 0.34885 = $30,000 × 0.01933 = $579.98/month
Total paid: $579.98 × 60 = $34,799
Total interest: $34,799 − $30,000 = $4,799
How Down Payment and Trade-in Affect Your Loan
Your loan amount is calculated as: Vehicle Price + Sales Tax − Down Payment − Trade-in Equity
Trade-in equity = Trade-in value minus any amount you still owe on the trade-in vehicle. If you owe more than your trade-in is worth (negative equity, sometimes called being "underwater"), that negative equity gets rolled into your new loan, increasing the amount you finance. This is common and worth understanding before you sign.
Vehicle price: $35,000
Sales tax (6%): $2,100
Down payment: $5,000
Trade-in value: $8,000, amount owed on trade-in: $10,000
Trade-in equity: $8,000 − $10,000 = −$2,000 (negative)
Loan amount = $35,000 + $2,100 − $5,000 − (−$2,000)
Loan amount = $35,000 + $2,100 − $5,000 + $2,000 = $34,100
In this scenario, the $2,000 in negative equity on the trade-in is rolled into the new loan, increasing what you finance. This means you'd be paying 6% APR interest on that negative equity for the duration of the new loan.
What Loan Term Does to Total Interest: A Comparison
Longer loan terms lower the monthly payment but dramatically raise the total interest you pay. Here's the comparison on a $30,000 loan at 7% APR:
48 months: $718/month | Total interest: $4,447
60 months: $594/month | Total interest: $5,640
72 months: $512/month | Total interest: $6,861
84 months: $453/month | Total interest: $8,027
Difference between 36-month and 84-month: $4,705 in extra interest for the convenience of a $473/month lower payment.
The 84-month loan cuts the monthly payment by 51% compared to the 36-month loan — but triples the total interest paid. There's another hidden cost: a 7-year loan means you're paying for a car for much longer than it holds its peak reliability, and you may owe more than the car is worth for years.
APR vs. Interest Rate
For most auto loans, the APR (Annual Percentage Rate) and the stated interest rate are very close — unlike mortgages, which have significant closing costs that widen the gap. However, dealer-arranged financing sometimes bundles in documentation fees or "finance reserve" markups that raise the effective cost. Always ask for the APR and compare it across at least two lenders (your bank or credit union, plus the dealership) before committing.
As of 2025–2026, average new car loan APRs range from approximately 5–8% for borrowers with good credit (700+) and 10–18% for subprime borrowers. Used car loans typically carry higher rates (1–3% above new car rates) because used cars carry more collateral risk for lenders.
What This Calculator Does Not Include
- Sales tax handling varies by US state: Some states tax only the price minus trade-in value; others tax the full vehicle price. Some dealer fees are taxable; others are not. This calculator applies the tax rate to the vehicle price only as an approximation.
- Registration and title fees: These vary by state and vehicle weight. They're typically a few hundred dollars and are sometimes financed into the loan but not reflected here.
- Dealer fees: Documentation fees ("doc fees"), dealer add-ons, and extended warranties increase your effective cost. These can be significant — doc fees alone range from $0 to $800+ depending on the state.
- GAP insurance: If you finance most of the vehicle price, your insurer's payout in a total loss may be less than your loan balance (since cars depreciate faster than loans are paid down). GAP insurance covers this difference. It's worth considering for high-LTV loans.
- Prepayment penalties: Rare in auto loans but occasionally present — check your loan agreement before making extra principal payments.
Frequently Asked Questions
Your monthly auto loan payment uses the standard amortization formula: M = P × r(1+r)^n / ((1+r)^n − 1), where P is the loan amount (vehicle price plus tax minus down payment and trade-in equity), r is your monthly interest rate (APR ÷ 12 ÷ 100), and n is the total number of monthly payments. Each payment covers the interest that accrued since your last payment, with the remainder reducing your principal balance. The payment is structured so the balance reaches exactly zero at the last payment.
A longer term lowers the monthly payment but significantly raises total interest paid. On a $30,000 loan at 7% APR: a 36-month term costs $3,322 in total interest; an 84-month term costs $8,027 — an extra $4,705 for the same car. Additionally, longer-term borrowers are "underwater" (owe more than the car is worth) for a longer period, which is a financial risk if you need to sell or the car is totaled. Choose the shortest term you can comfortably afford.
A common guideline is 20% down on a new car and 10% on a used car. A larger down payment: (1) reduces your monthly payment and total interest, (2) reduces the risk of negative equity since cars depreciate rapidly (new cars lose 15–25% of value in the first year), and (3) may qualify you for a lower APR since the loan-to-value ratio is lower. If you can't put 20% down, try to at least cover the first year's expected depreciation to avoid going underwater immediately.
As of 2025–2026, average new car loan APRs are approximately 5–8% for borrowers with good credit (FICO 700+). Excellent credit (750+) may qualify for 4–6% at credit unions or banks. Subprime borrowers (below 620) typically face 12–18% or higher. Used car loans generally run 1–3% higher than new car rates because of depreciation and collateral risk. Always compare the financing from your bank or credit union against the dealership's offer — dealers earn a margin on financing and may mark up the rate from what the lender would charge directly.
Negative equity (being "underwater" or "upside down") means you owe more on your current vehicle than it's worth as a trade-in. For example, if your car is worth $8,000 but you still owe $10,000 on it, you have $2,000 in negative equity. When you trade in a car with negative equity, that shortfall is typically rolled into your new loan — you borrow more than the new car costs. This increases your monthly payment and interest cost, and means you start the new loan underwater. It's a cycle worth breaking by paying off the old loan before trading, making a larger down payment, or simply waiting.