Car loans are calculated using a standard amortization formula that combines your loan amount, interest rate, and loan term to produce a fixed monthly payment. Every payment stays the same dollar amount, but the split between interest and principal shifts over time. Understanding how this math works helps you see exactly where your money goes each month and how choices like loan term, down payment, and credit score change what you ultimately pay.
The Monthly Payment Formula
Lenders use this formula to calculate your fixed monthly payment:
Monthly Payment = Loan Amount × [i × (1 + i)^n] / [(1 + i)^n – 1]
There are two variables you need to plug in. The first is “i,” your monthly interest rate, which is your annual rate divided by 12. If your annual rate is 6%, your monthly rate is 0.5% (or 0.005 as a decimal). The second is “n,” the total number of monthly payments. A five-year loan has 60 payments (5 × 12), and a six-year loan has 72.
Here’s a concrete example. Say you borrow $30,000 at 6% for five years. Your monthly rate is 0.005, and your number of payments is 60. Running the formula gives you a monthly payment of about $580. Over the full 60 months, you’d pay roughly $34,800 total, meaning about $4,800 goes to interest on top of the $30,000 you borrowed.
What Makes Up Your Loan Amount
The number that goes into the formula isn’t just the sticker price of the car. Your total loan amount, often called the principal, includes the vehicle price minus your down payment and trade-in value, plus several additional costs. Sales tax is the biggest one, and it can add thousands of dollars depending on where you live. Documentation fees, title and registration charges, and any dealer add-ons you agree to (like an extended warranty, GAP insurance, or credit insurance) also get rolled in.
Every dollar added to the principal generates interest over the life of the loan. A $2,000 extended warranty financed at 7% over five years doesn’t cost you $2,000. It costs closer to $2,400 once interest is factored in. That’s why it’s worth scrutinizing optional products before signing. If you don’t need them, removing them shrinks your principal and reduces total interest.
Simple Interest vs. Precomputed Interest
Most auto loans today use simple interest, which means the interest you owe each month is calculated based on your actual outstanding balance at the time your payment is due. If you pay extra one month, your balance drops faster, and the next month’s interest charge is smaller. This rewards you for making additional payments or paying early.
A less common method is precomputed interest. With precomputed loans, the lender calculates all the interest you’d owe over the full term upfront and adds it to the principal before splitting everything into monthly payments. Making extra payments on a precomputed loan doesn’t reduce the interest you owe because it was already baked in from the start. If you’re offered a precomputed loan, be aware that paying ahead won’t save you money the way it would with a simple interest loan.
How Amortization Shifts Your Payments
Even though your monthly payment stays the same, the portion going to interest versus principal changes with every payment. Early in the loan, a larger share of each payment covers interest. As you pay down the balance, the interest portion shrinks and more of your payment goes toward principal. This pattern is called amortization.
In practical terms, your balance decreases slowly at first and then drops more quickly toward the end of the loan. On a $30,000 loan at 6% for five years, your first payment of $580 might include $150 in interest and $430 toward principal. By payment 50, the interest portion might be only $17, with $563 going to principal. This front-loading of interest is why paying extra early in the loan has the biggest impact on total cost. Every extra dollar you put toward principal in the first year or two eliminates interest that would have accumulated over the remaining term.
How Your Interest Rate Is Determined
Your credit score is the single biggest factor in the rate a lender offers you, and the gap between credit tiers is substantial. Based on Q3 2025 data from Experian, here’s what borrowers are paying on average:
- Super prime (781 to 850): 4.66% on new cars, 7.70% on used
- Prime (661 to 780): 6.27% on new cars, 9.98% on used
- Near prime (601 to 660): 9.57% on new cars, 14.49% on used
- Subprime (501 to 600): 13.17% on new cars, 19.42% on used
- Deep subprime (300 to 500): 16.01% on new cars, 21.85% on used
The difference is dramatic in dollar terms. On a $25,000 loan over five years, a borrower at 5% pays about $3,300 in total interest. A borrower at 14% pays roughly $9,700. Same car, same loan term, but $6,400 more out of pocket.
Used cars consistently carry higher rates than new ones because they’re riskier collateral for lenders. Most lenders won’t finance vehicles over 10 years old or with more than 100,000 miles, and they may impose additional limits based on the vehicle’s brand, condition, or value.
How Loan Term Affects Total Cost
Stretching a loan from 48 months to 72 months lowers your monthly payment but increases the total interest you pay. That’s because you’re carrying a balance for an extra two years, and every month that balance exists, it generates interest.
Consider a $28,000 loan at 7%. Over 48 months, your payment is about $670 and you pay $4,150 in total interest. Stretch the same loan to 72 months and your payment drops to roughly $478, but total interest climbs to $6,400. You save $192 a month but pay $2,250 more overall. Longer terms also increase the risk of being “upside down,” meaning you owe more than the car is worth, because the balance decreases slowly while the car depreciates.
The Effect of a Down Payment
A down payment reduces your principal before the formula ever runs, which lowers both your monthly payment and total interest. Putting $5,000 down on a $30,000 car means you’re financing $25,000 instead of $30,000. At 6% over five years, that $5,000 down payment saves you roughly $800 in interest on top of reducing your monthly payment by about $97.
A trade-in works the same way mathematically. If the dealer credits you $8,000 for your old car, that’s $8,000 subtracted from the amount you need to finance. Just be sure the trade-in value is applied before any add-ons or fees are calculated, so you get the full benefit.
Putting It All Together
To estimate your own car loan, you need four numbers: the total amount financed (price minus down payment, plus taxes and fees), the annual interest rate, the loan term in months, and whether the loan uses simple or precomputed interest. Plug the first three into the amortization formula, or use any online auto loan calculator, and you’ll get your monthly payment. From there, multiply the monthly payment by the number of months and subtract the principal to see your total interest cost.
The levers you can pull to reduce that cost are straightforward: improve your credit score before applying, make a larger down payment, choose a shorter loan term, skip optional add-ons you don’t need, and if you have a simple interest loan, make extra payments toward principal whenever you can. Each of those moves shrinks the numbers the formula works with, and smaller inputs mean less interest paid over the life of the loan.