Auto Loan Calculator

How to Calculate Monthly Car Loan Payments & Total Financing Costs

An auto loan is an amortizing installment loan: you borrow a fixed amount, agree to a fixed annual percentage rate (APR), and repay it in equal monthly payments over a set term. Each payment covers the interest accrued that month first, and the remainder reduces your principal balance. Because the principal shrinks with every payment, the interest portion of each payment shrinks too — a process called amortization. Our auto loan calculator applies the standard amortization formula M = P × r(1+r)ⁿ ÷ ((1+r)ⁿ − 1) to give you an exact monthly payment, total interest, and full loan cost in real time.

It helps to understand the difference between simple interest and an amortizing vehicle loan. Most car loans are simple-interest amortizing loans, meaning interest is calculated daily on the outstanding principal rather than compounded onto the balance. That is good news: making extra payments or paying bi-weekly directly reduces principal and the interest you owe. The APR you are quoted already bundles the interest rate plus most finance charges, so it is the number to compare across lenders — not the advertised "interest rate."

Dealer APR vs. credit union financing is one of the biggest cost levers in a car purchase. Dealers often mark up the lender's buy rate (the wholesale rate a bank offers the dealer) and keep the difference as compensation, so a dealer's "special" APR can still cost more than a pre-approval from a credit union or bank. Always arrive with a pre-approved APR in hand — it becomes a ceiling the dealer has to beat, and it removes pressure from the finance office. A difference of just 1 percentage point on a $30,000 loan over 60 months is roughly $800 in interest.

Loan term length has an outsized impact on total interest. Stretching a loan from 48 to 72 months lowers the monthly payment but dramatically increases the total interest paid, and it raises the risk of negative equity — owing more than the car is worth as it depreciates. Use the comparison table below to see the trade-off clearly before you sign.

Auto Loan Terms Compared: 48, 60, and 72-Month Financing

The table below compares a $25,000 vehicle purchase at 6.5% APR across three common terms, assuming no down payment or trade-in. Monthly payment, total interest, and total cost all change meaningfully with the term you choose.

Notice that moving from 48 to 72 months cuts the monthly payment by about $174 but adds roughly $1,755 in interest over the life of the loan. Longer terms also create negative equity, or "gap," risk: a new car can lose 20% or more of its value in the first year. On a 72-month loan with a small down payment, you can easily owe more than the vehicle is worth for the first several years — a serious problem if the car is totaled or you want to trade it in. Gap insurance covers the difference between what you owe and what the car is worth, and is worth considering on any loan over 60 months with less than 20% down.

Impact of Down Payments & Trade-In Equity

A down payment and a trade-in both reduce the amount you finance, which lowers your monthly payment and the total interest you pay. The widely cited 20/4/10 rule is a useful affordability benchmark: put at least 20% down, finance for no more than 4 years (48 months), and keep total monthly vehicle expenses — payment, insurance, fuel, and maintenance — under 10% of your gross monthly income. Following it protects you from negative equity and keeps transportation costs from crowding out other financial goals.

How you finance a $10,000 used car loan differs from financing a new car. Used vehicles typically carry slightly higher APRs than new cars because lenders view them as higher risk, but their lower purchase price means you can often finance them over a shorter 36–48 month term without straining your budget. A $10,000 used car loan at 8% APR over 48 months costs about $244 a month and roughly $1,718 in total interest — far cheaper than stretching a larger new-car loan. Use the calculator above to model your exact scenario, including down payment and trade-in value.

Trade-in equity works like a down payment: it reduces the financed amount. But beware of negative trade-in equity (being "upside down" on your current car). If you owe more on your trade-in than it is worth, the dealer rolls that negative balance into the new loan — increasing the new loan's principal and the interest you pay. Always check your current payoff amount against the trade-in value before negotiating.

All figures are estimates for informational and planning purposes only and do not constitute financial, tax, or mortgage advice. Confirm final rates, taxes, and insurance with your lender, county assessor, and a licensed insurance agent.