Auto Loan Guide: How Car Payments Work

An auto loan turns the price of a car into equal monthly payments over a fixed term — usually 36 to 72 months. The math is the same amortization formula as any installment loan, but two car-specific twists make auto loans especially tricky: depreciation (the car loses value faster than you pay down the loan) and APR vs. interest rate (fees get folded in).

The Monthly Payment Formula

M = P × [ r(1 + r)ⁿ ] / [ (1 + r)ⁿ − 1 ]

M — monthly payment
P — amount financed (price − down payment − trade-in)
r — monthly rate (APR ÷ 12)
n — total months (years × 12)

Example: a $30,000 car, $4,000 down, 6% APR for 60 months. P = $26,000, r = 0.005, n = 60. M = 26,000 × [0.005(1.005)⁶⁰] / [(1.005)⁶⁰ − 1] ≈ $503/month. Total paid: $503 × 60 = $30,180, so interest ≈ $4,180.

APR vs. Interest Rate

The interest rate is the cost of borrowing; the APR (Annual Percentage Rate) bundles in loan fees, so APR is always ≥ interest rate. Always compare loans by APR — it's the true annual cost.

⚠️ The 72- and 84-month trap

Longer terms drop the payment sharply, but cars depreciate ~20% in year one and 60% over 5 years. A 72-month loan can leave you "underwater" — owing more than the car is worth — for years. That's disastrous if the car is totaled or you want to sell.

Depreciation Outpaces Principal

In the early years of a loan, the car's value falls faster than your balance drops. On a $30K car financed over 72 months, by month 24 you might owe $22,000 on a car worth $19,000. Gap insurance covers this difference if the car is totaled — strongly recommended for long loans or small down payments.

How to Pay Less

Put 20% down. Avoids negative equity and lowers every payment.
Keep the term ≤ 60 months. Saves thousands in interest and prevents being underwater.
Compare dealer vs. bank vs. credit union. Credit unions often beat dealer rates.
Don't finance taxes & fees. Pay them upfront if you can.

Put It Into Practice

Enter the car price, down payment, trade-in, APR, and term to see your monthly payment, total interest, and an amortization schedule.