Auto Loan Calculator

How Is a Car Loan Payment Calculated?

An auto loan uses the same standard amortization formula as any installment loan. The lender starts with the net amount financed — vehicle price, minus your down payment and trade-in, plus any taxes and fees rolled into the loan — and applies a fixed monthly interest rate to compute a level payment that fully retires the balance by the final scheduled date.

• M — monthly payment
• P — net amount financed (after down payment and trade-in)
• r — monthly rate (annual rate ÷ 12)
• n — total months (term in years × 12)

Down Payment and Trade-in: How Much They Really Save

Every dollar of down payment or trade-in value reduces the principal directly, which lowers both the monthly payment and the total interest paid. But the benefit goes beyond arithmetic: starting with more equity means you are less likely to be "underwater" — owing more on the loan than the car is worth — which is a serious risk in the first two years of any auto loan, when depreciation is steepest.

For a typical new vehicle, 20% down is a widely cited target because it roughly matches the first-year depreciation hit of 15–25%. Used vehicles depreciate more slowly, so smaller down payments carry less underwater risk. As a rule of thumb, any down payment that keeps your loan-to-value ratio at or below 100% from day one is financially sound.

The Hidden Cost of a Longer Auto Loan Term

Dealers and finance offices often push 72- or 84-month loans because the low monthly payment feels affordable. But the total interest cost of a 7-year auto loan can be 2–3× that of a 3-year loan on the same vehicle, even before accounting for the depreciation problem: by year 5 or 6, many vehicles are worth less than the outstanding loan balance, leaving the borrower liable for the difference if the car is totalled or needs to be sold.

The term comparison table above shows this trade-off for your specific loan amount and rate. As a general rule, try to finance a used car for no more than 48 months and a new car for no more than 60 months to stay within a reasonable depreciation curve.

New vs. Used Car Loan Rates

New car loans typically carry lower interest rates than used car loans because lenders consider new vehicles a lower-risk collateral — the value is well-established and the car has a full manufacturer warranty. The rate difference is usually 1–3 percentage points, but widens significantly as a used vehicle ages. Vehicles older than 7 years may not qualify for conventional auto financing at all and instead require a personal loan, which typically carries an even higher rate.

When comparing a new versus used purchase, factor in the higher interest rate on a used vehicle. The used car's lower sticker price does not always mean a lower total cost once financing is included. The reverse is also true: a certified pre-owned vehicle with a factory-backed rate can sometimes be cheaper to finance than a private-party used vehicle, even at a higher price.

About These Parameters

Vehicle Price

The agreed purchase price of the vehicle, before any down payment, trade-in, or incentive is applied. For new cars this is typically the negotiated selling price, not the MSRP. For used cars it is the asking or agreed price. This figure is the starting point from which down payment, trade-in, taxes, and fees are added or subtracted to determine the net amount financed.

Down Payment

Cash paid directly to the dealership or seller at the time of purchase. It reduces the financed principal dollar-for-dollar and lowers your monthly payment. More importantly, a larger down payment reduces the risk of being underwater on the loan in the first few years, when a vehicle depreciates fastest. A target of 10–20% of vehicle price is a common rule of thumb.

Trade-in Value

The dealer's agreed value for your current vehicle, applied as a credit toward the purchase. It functions identically to a down payment in the financing math — it reduces the amount financed. The actual trade-in offer you receive may differ from private-sale value; check resources like KBB or Edmunds before accepting a dealer offer.

Annual Interest Rate

The yearly rate charged on the outstanding loan balance. This is the rate quoted by the lender, not the APR (which includes fees and other finance charges). Auto loan rates depend heavily on your credit score, the vehicle age, the loan term, and whether you are financing through a bank, credit union, or dealership. Credit unions typically offer 0.5–1.5% lower rates than dealership financing.

Loan Term

The number of months over which you repay the loan. Common auto loan terms are 24, 36, 48, 60, 72, and 84 months. Longer terms lower the monthly payment but increase total interest and may leave you owing more than the car is worth mid-loan. The term comparison table shows the trade-off for your specific loan amount and rate.

Sales Tax

Most states charge sales tax on vehicle purchases, typically 4–10% of the purchase price. Some buyers pay this upfront at registration; others roll it into the financed amount, which then incurs interest over the life of the loan. Rolling in a 7% tax on a $25,000 vehicle ($1,750) at 7% interest over 5 years adds roughly $330 in interest — a small but real cost to financing your tax bill.

Fees & Other Costs

Documentation fees, title and registration, dealer processing charges, and any optional add-ons (extended warranty, GAP insurance, paint protection) that are rolled into the loan. These are often negotiable, and paying them out of pocket rather than financing them saves the interest cost on top. Always ask for an itemized breakdown before agreeing to roll any fee into the loan.

Frequently Asked Questions