Auto Loan Calculator

Free auto loan calculator with trade-in, down payment, tax and fees. See your monthly payment, total interest, and the 20/4/10 rule.

The dealership talks in monthly payments. You shouldn’t.

Walk into any dealership and the conversation bends toward one number: the monthly payment. Can we get you to $450 a month? Of course they can. Stretch the loan to 84 months, shrink the down payment, roll in the fees, and any car fits any payment. The payment is the least informative number in the building. What matters is the total cost, the interest, and whether the car fits your income. That’s what this auto loan calculator with trade-in shows you before you ever shake a salesperson’s hand.

Your payment isn’t based on the sticker price

The monthly payment is calculated on the net financed amount, what you actually borrow after everything nets out:

Net financed = price − down payment − trade-in + sales tax + fees

Sales tax usually applies to the price minus the trade-in (most states give you that credit), and the trade-in counts at its net value: what it’s worth minus anything you still owe on it. Owe more than it’s worth? That shortfall gets added to your new loan, which is how people end up financing two cars while driving one.

The amortization formula, with real numbers

Every US lender uses the same standard fixed-rate amortization formula:

Monthly payment = P × r(1+r)ⁿ ÷ ((1+r)ⁿ − 1)

where P is the net financed amount, r is the monthly rate (APR ÷ 12), and n is the term in months. Let’s run a real deal through it. You negotiate a car to $35,000, put $7,000 down, and get a $4,000 trade-in credit. Sales tax at 6.5% applies to the net $31,000, adding $2,015. Net financed: $35,000 − $7,000 − $4,000 + $2,015 = $26,015.

At 6.50% APR for 60 months, the monthly rate is 0.0054167 and the formula gives a payment of $509.01. Over 60 months you pay $30,540.60 total, which means $4,525.60 in interest, and your all-in vehicle outlay is $41,540.60 once the down payment and trade-in are counted. Plug the same structure into the calculator above with your own price, rate, and term.

Notice the shape of amortization: each payment covers that month’s interest first, and the rest reduces principal. Early on, most of your $509 is interest. As the balance falls, more of each payment attacks principal. That’s why extra principal payments early in the loan save disproportionate interest. And if this loan is one of several debts you’re carrying, the debt snowball vs avalanche calculator shows where a car payment belongs in the payoff order against credit cards and other loans.

How much car can you actually afford? The 20/4/10 rule

Before falling for a windshield number, run the 20/4/10 rule that financial planners have recommended for years:

20% down. At least a fifth of the price upfront. New cars lose roughly 20% of their value in year one, so this keeps you from going underwater immediately.
4-year term max. Finance for no more than 48 months. If you need 72 or 84 months to afford the payment, the car is too expensive. Shorter terms also mean you pay the car off while it still has real value.
10% of income. Your total monthly car costs, payment plus insurance, fuel, and maintenance, stay under 10% of gross monthly income.

That third point is the one everyone skips. On a $60,000 salary, 10% is $500 a month for everything car-related. If insurance and gas run $200, only $300 is left for the loan payment. Over 4 years at a typical rate, that supports a loan around $13,000, plus your down payment. Sobering, but that’s the point. The rule exists to keep a car from quietly eating the rest of your financial life.

60 vs 72 months: what the extra two years really cost

Dealers love long terms because they shrink the payment. Take our $26,015 loan at 6.5%. At 60 months: $509/month, $4,526 in total interest. Stretch it to 72 months: $437/month, but $5,471 in total interest. You “save” $72 a month and pay $945 more in interest, while spending two extra years owing more than the car is worth. Longer terms often carry higher rates too, which widens the gap further. The payment went down. The car got more expensive. That’s the trade, stated plainly. Money handed to the bank as interest is also money that never gets invested, and the compound interest calculator shows what even a few hundred dollars of it would have become over a couple of decades.

Trading in a car you still owe money on

If your trade-in is worth $12,000 and you owe $15,000, you have $3,000 of negative equity. That $3,000 doesn’t vanish, it gets rolled into the new loan, so you’re financing $3,000 of a car you no longer own, at the new car’s rate, for the new car’s term. Do this twice in a row and the hole gets deep. The cleanest move is usually to pay the difference in cash or wait until you’re right-side up. At minimum, know the number before you negotiate, because the dealership will happily bury it in the monthly payment where you’ll never see it.

What moves your rate

Average auto loan rates cluster by credit: excellent credit (750+) typically sees roughly 5 to 7% on new cars, good credit (700s) more like 7 to 10%, and used-car loans run a couple of points higher than new. Your bank or credit union will usually beat dealer financing, so get pre-approved before you visit. That pre-approval is also your negotiating weapon: the dealer has to beat a real number, not a vibe. Watch out for 0% APR promotions, which often require giving up a cash rebate. Run both scenarios in the calculator. Sometimes the rebate plus your bank’s rate wins.

Auto loan FAQs

How is my auto loan monthly payment calculated?

Lenders use the standard amortization formula on your net financed amount: monthly payment = P × r(1+r)ⁿ ÷ ((1+r)ⁿ − 1), where P is amount financed, r is APR ÷ 12, and n is months. The net financed amount is price minus down payment minus trade-in, plus sales tax and fees.

How much car can I afford?

Use the 20/4/10 rule: at least 20% down, a loan term of 4 years or less, and total monthly car costs (payment, insurance, fuel, maintenance) under 10% of gross monthly income. If the car you want needs a longer loan to fit, it’s too expensive.

Is a 72-month car loan a bad idea?

Usually, yes. Compared with 60 months on the same loan, 72 months lowers the payment but adds roughly a thousand dollars or more in interest and keeps you underwater (owing more than the car’s worth) far longer. Cars depreciate fastest early, exactly when a long loan pays down slowest.

What does “upside down” on a car loan mean?

It means you owe more than the car is worth. It happens with small down payments, long terms, and fast depreciation. If you trade in while upside down, the shortfall rolls into your next loan. A 20% down payment and a shorter term are the main defenses.

Should I take the 0% APR deal or the cash rebate?

Do the math both ways, because 0% financing usually means forfeiting the rebate. Take the car’s price minus the rebate, finance it at your bank’s rate, and compare total cost against the 0% offer on the full price. The calculator above handles both scenarios.

Does my trade-in reduce the sales tax I pay?

In most states, yes: sales tax applies to the purchase price minus the trade-in value. On a $35,000 car with a $4,000 trade-in, you’re taxed on $31,000 instead of $35,000. A few states don’t offer this credit, so check your state’s rules.

This calculator provides an educational estimate based on the numbers you enter. Actual loan terms, rates, taxes, and fees vary by lender and state. It is not financial advice; consult a qualified professional before borrowing.