How to Calculate a Car Loan Payment: The Amortization Formula

The amortization formula, step by step

Every fixed-rate car loan uses the same equation: M = P × r × (1 + r)^n ÷ ((1 + r)^n − 1). P is what you borrow, r is the monthly rate, and n is the total number of payments. The exponent (1 + r)^n is where compounding lives — it is why doubling the term does not simply halve the payment.

Take a $25,000 loan at 7% over 60 months. The monthly rate is 0.07 ÷ 12 = 0.005833, and (1.005833)^60 ≈ 1.4176. The payment works out to 25,000 × 0.005833 × 1.4176 ÷ 0.4176 ≈ $495. Over 60 months that is $29,700 repaid, so around $4,700 is interest — the true cost of borrowing on top of the car.

How term and rate change the payment

Stretching the same $25,000 loan from 60 to 72 months drops the payment to about $426, which looks appealing. But you now make 72 payments, and total interest climbs to roughly $5,700 — about $1,000 more — because you owe the balance for longer. A shorter term costs more each month but far less overall.

The interest rate matters just as much. On the same 60-month loan, moving from 7% to 4% cuts the payment from about $495 to $460 and the total interest from $4,700 to roughly $2,600. Because rate depends heavily on your credit score, checking your score before you shop can be worth thousands.

Beyond the payment: the true cost of the car

The loan payment is only part of ownership. Insurance, fuel, maintenance and registration add up quickly, and a common rule of thumb keeps total car costs under about 15% to 20% of take-home pay. A bigger down payment shrinks the principal and the interest, and it protects you from owing more than the car is worth as it depreciates.

Watch out for the annual percentage rate (APR) versus the nominal rate: the APR folds in fees, so it reflects the real cost better. And beware zero-interest offers that require giving up a cash rebate — sometimes taking the rebate and a normal loan is cheaper overall.

Frequently asked questions

Does a bigger down payment lower my monthly payment?

Yes. A down payment reduces the principal P, so both the monthly payment and the total interest fall. Putting $3,000 more down on a $25,000 loan at 7% over 60 months cuts the payment by about $59 a month.

What is the difference between APR and interest rate?

The interest rate is the cost of borrowing the principal alone. The APR adds in mandatory fees, so it is usually higher and gives a truer picture. Compare offers by APR, not just the headline rate.

Should I choose a longer term to lower the payment?

A longer term lowers the monthly payment but raises total interest, and you risk owing more than the car is worth. Pick the shortest term whose payment still fits comfortably in your budget.

Can I pay off a car loan early to save interest?

Usually yes. Because interest accrues on the remaining balance, extra payments reduce future interest. Check first for any prepayment penalty, and confirm extra money is applied to the principal.