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Understanding Your Auto Loan

An auto loan is a secured loan used to purchase a vehicle. The car itself serves as collateral, which means the lender can repossess it if you stop paying. Because that lowers the lender's risk, auto loans usually carry lower interest rates than unsecured personal loans. This calculator estimates your monthly payment and total cost from the amount financed, the interest rate, and the loan term, and it accounts for your down payment, trade-in, and sales tax.

How the Amount Financed Is Calculated

The amount financed is the figure the amortization formula is actually applied to. It equals the vehicle price plus applicable sales tax, minus your down payment and any trade-in value. Once that number is set, the monthly payment is found with the standard formula M = P[r(1+r)^n] / [(1+r)^n − 1], where P is the amount financed, r is the monthly rate (annual rate ÷ 12), and n is the number of monthly payments (the term in months). Reducing the amount financed through a larger down payment or trade-in is one of the most effective ways to lower both your monthly payment and your total interest cost.

A Worked Example

Suppose you buy a vehicle priced at $30,000 in a state with a hypothetical 6% sales tax, put $3,000 down, and have no trade-in. Sales tax adds $1,800, so the amount financed is $28,800. At a hypothetical 5.5% annual rate over 60 months, the monthly payment works out to roughly $550, and you would pay about $4,200 in interest over the life of the loan. Increasing the down payment or shortening the term would lower both of those figures. Adjust the inputs above to model your own scenario.

The Term Length Trade-Off

Stretching a car loan to a longer term is tempting because it shrinks the monthly payment, but it has two downsides. First, you pay considerably more total interest. Second, because vehicles depreciate quickly, a long term keeps you upside down (owing more than the car is worth) for a longer stretch, which is risky if the car is totaled or you need to sell. A shorter term costs more each month but builds equity faster and reduces total interest.

New vs. Used Car Loans

New car loans typically offer lower interest rates and longer available terms, and manufacturers sometimes provide promotional financing. Used car loans may carry slightly higher rates, but the lower purchase price often results in a smaller loan and lower overall cost. When deciding, look beyond the payment to the total cost of ownership, including depreciation, insurance, fuel, and maintenance.

How Each Input Changes Your Payment

Every field in this calculator pushes your payment in a predictable direction, so it helps to understand the levers before you negotiate:

• Vehicle price: The starting point for everything. A lower negotiated price reduces both the loan and the sales tax charged on it.
• Down payment: Cash paid upfront that directly lowers the amount financed, your payment, and total interest.
• Trade-in value: Works like additional down payment by reducing the financed balance. In some states it also lowers the taxable amount.
• Interest rate: The price of borrowing. Improving your credit or comparing lenders can move this more than you might expect.
• Loan term: A longer term lowers the monthly payment but raises total interest and extends the period of negative equity.
• Sales tax: Added to the financed amount in most states, so a higher tax rate quietly increases your payment.

Avoiding Common Pitfalls

The most common mistake at a dealership is negotiating around the monthly payment instead of the total price. A salesperson can hit almost any target payment by extending the term, which can leave you paying far more overall. Focus on the out-the-door price, the interest rate, and the total interest figure shown above. Be cautious about rolling negative equity from an old loan into a new one, and treat optional add-ons such as extended warranties as separate decisions rather than small line items buried in the payment.

When to Use a Different Calculator

This calculator is purpose-built for vehicle purchases because it handles trade-ins and sales tax. For a general fixed-rate loan without those extras, use the loan calculator. To study how additional principal payments could shorten the loan, the amortization calculator lets you add an extra monthly payment.