What Is an Auto Loan?
An auto loan is a secured installment loan used to finance the purchase of a vehicle. The lender pays the dealer directly, and you repay the lender in fixed monthly installments over a set term — typically 24 to 84 months. The vehicle itself serves as collateral, which is why auto loan rates are generally lower than unsecured personal loan rates.
Auto loans are characterized by three key numbers: the loan amount (vehicle price minus down payment and trade-in), the Annual Percentage Rate (APR), and the loan term in months. These three inputs entirely determine your monthly payment and total interest cost.
How to Use This Calculator
- Vehicle Price: Enter the full sticker price or agreed purchase price of the car.
- Down Payment: The cash you're paying upfront. A larger down payment reduces your loan and monthly payment.
- Trade-In Value: If you're trading in a vehicle, its value is subtracted from the loan amount — effectively acting as an additional down payment.
- Loan Term: Choose how many months you'll take to repay. Shorter terms mean higher payments but less total interest.
- APR: Your Annual Percentage Rate. This is the annual interest rate including fees. Get pre-approved quotes from banks and credit unions before visiting a dealer.
- Sales Tax: Enter your state/local sales tax rate if you're rolling it into the loan. Many buyers pay tax at signing to avoid interest on it.
The Auto Loan Formula
The monthly payment is calculated using the standard amortization formula:
Loan Amount = Vehicle Price − Down Payment − Trade-In + (Vehicle Price × Sales Tax%)
Monthly Payment (M) = L × [r(1+r)^n] / [(1+r)^n − 1]
Where:
L = Loan amount
r = Monthly interest rate (APR ÷ 12 ÷ 100)
n = Number of months
The total interest paid equals (Monthly Payment × n) − Loan Amount. The amortization table shows how each payment is split between principal reduction and interest — in the early months, most of your payment goes to interest; by the end of the loan, most goes to principal.
Real-World Example
Let's say you're buying a $35,000 car with a $5,000 down payment, no trade-in, at 7% APR for 60 months, with 8% sales tax financed:
- Sales tax = $35,000 × 8% = $2,800
- Loan amount = $35,000 − $5,000 + $2,800 = $32,800
- Monthly rate r = 7% ÷ 12 ÷ 100 = 0.005833
- Monthly payment = $32,800 × [0.005833 × (1.005833)^60] / [(1.005833)^60 − 1] = $648.52
- Total paid = $648.52 × 60 = $38,911
- Total interest = $38,911 − $32,800 = $6,111
Paying the $2,800 sales tax upfront instead of financing it would save roughly $520 in interest over the loan term — a meaningful saving for a simple choice.
Comparing Loan Terms on the Same Car
Using $30,000 at 7% APR with no tax or trade-in:
- 36 months: $927/mo — $3,368 total interest
- 48 months: $717/mo — $4,421 total interest
- 60 months: $594/mo — $5,528 total interest
- 72 months: $513/mo — $6,936 total interest
- 84 months: $453/mo — $8,038 total interest
The 84-month loan costs more than twice the interest of the 36-month loan, and keeps you in debt for 7 years on a depreciating asset. Financial advisors generally recommend keeping auto loans to 48–60 months maximum.
Tips for Getting the Best Auto Loan
- Get pre-approved before visiting the dealer. A pre-approval from your bank or credit union gives you a rate benchmark and negotiating power.
- Check your credit score first. Even a 20–30 point improvement in your credit score can lower your APR by 1–2%, saving hundreds over the loan.
- Negotiate the car price separately from financing. Dealers sometimes obscure the total cost by focusing on monthly payment. Agree on the purchase price first, then discuss financing.
- Avoid extras rolled into the loan. Extended warranties and paint protection rolled into a financed amount cost significantly more over time due to interest.
- Consider a shorter loan. If you can afford a 48-month payment comfortably, choose 48 over 60 — the interest savings are substantial.
Frequently Asked Questions
How is my monthly car payment calculated?
Your monthly payment uses the standard amortization formula: M = L × [r(1+r)^n] / [(1+r)^n − 1], where L is the loan amount, r is the monthly rate (APR ÷ 12 ÷ 100), and n is the term in months. This ensures your loan is fully paid off with the final payment, with each installment covering both interest and principal.
Should I roll sales tax into my auto loan?
Rolling sales tax into the loan is convenient but means you'll pay interest on that amount for the entire loan term. On $2,500 of tax at 7% for 60 months, you'd pay about $463 extra in interest. If you can afford to pay tax upfront at signing, it saves money. If cash is tight, financing the tax is a reasonable choice.
What is a good APR for a car loan?
For borrowers with excellent credit (750+ FICO), good new-car rates are typically below 6% and used-car rates below 9%. Average rates in 2025 are 7–10% for new cars and 10–14% for used. Credit unions usually offer lower rates than banks or dealer financing. Always shop at least 3 lenders before accepting any loan offer.
How much does a longer loan term actually cost?
On a $25,000 loan at 7% APR: a 48-month term costs $3,705 in total interest; a 60-month term costs $4,752; a 72-month term costs $5,724; and an 84-month term costs $6,704. The 84-month loan is nearly $3,000 more expensive than the 48-month loan — purely in interest on the same car purchase.
Is it better to make a larger down payment?
Yes, in almost every case. A larger down payment reduces your loan principal, which lowers both monthly payments and total interest. It also reduces the risk of being "underwater" (owing more than the car's value due to depreciation). A common guideline is 20% down for new cars and 10% for used. Even an extra $1,000 upfront can save several hundred dollars in interest over the life of the loan.