The Monthly Payment Formula
An auto loan is a standard amortizing loan — the same math behind a mortgage, just over a shorter term. Each month you pay a fixed amount that's split between interest (based on the remaining balance) and principal (which shrinks the balance). The formula for the fixed monthly payment is:
where L is the loan amount, r is the monthly interest rate (annual rate ÷ 12), and n is the number of monthly payments. The loan amount itself isn't just the sticker price — it's the price minus any down payment, minus your trade-in's net value, plus sales tax and fees if you choose to roll them into the loan rather than pay them upfront.
A Full Worked Example
Say you're buying a $50,000 vehicle with a $10,000 down payment, no trade-in, a 5% annual rate over 60 months, 7% sales tax, and $2,000 in title/registration fees paid upfront (not rolled into the loan):
- Loan amount = $50,000 − $10,000 = $40,000
- Sales tax = $50,000 × 7% = $3,500
- Upfront cash needed = $10,000 down + $3,500 tax + $2,000 fees = $15,500
- Monthly payment on $40,000 at 5%/60mo = $754.85
- Total of 60 payments = $45,290.96, meaning total interest paid = $5,290.96
- Total cost of ownership (price + interest + tax + fees) = $60,790.96
Run these exact numbers through the calculator above to see the full month-by-month breakdown.
How Trade-In Value Affects Your Sales Tax
In most states that charge sales tax on vehicle purchases, trading in your old car reduces the taxable amount — you're only taxed on the difference between the new car's price and your trade-in's value. For a $50,000 purchase with a $10,000 trade-in at an 8% tax rate, most states would calculate tax as:
A handful of states don't offer this reduction and tax the full purchase price regardless of trade-in — for the same example, that works out to $4,000, an $800 difference. Our calculator automatically applies the correct method based on the state you select.
Common Fees on a Car Purchase
| Fee | What It Covers |
|---|---|
| Sales Tax | State tax on the purchase; five states charge none at all |
| Document Fee | Dealer's charge for processing title/registration paperwork |
| Title & Registration | State fee to legally title and register the vehicle in your name |
| Destination Fee | Shipping the vehicle from factory to dealer, typically $900–$1,500 |
| Advertising Fee | Regional dealer marketing cost, sometimes folded into the sticker price |
Running costs outlast the loan. Our fuel cost calculator prices a journey, the gas mileage calculator works out consumption, and the mileage calculator totals distance for reimbursement.
If your dealer bundles taxes and fees into the loan itself rather than collecting them upfront, check the "Include taxes and fees in loan" box in the calculator so your loan amount and monthly payment reflect that correctly.
Dealership Financing vs. Direct Lending
You generally have two paths to financing a car: direct lending, where you secure a loan from a bank or credit union before you ever set foot in a dealership, or dealership financing, arranged through the dealer itself (often with a lender tied to the manufacturer). Walking in with a pre-approved direct loan gives you real negotiating leverage — the dealer has to beat your rate or lose the sale outright, rather than you being stuck accepting whatever financing they offer.
Getting a Better Deal
- Shop your rate first. Get quotes from at least two or three lenders before you negotiate price — a pre-approval is your strongest leverage.
- Negotiate the price separately from the payment. Dealers can make almost any monthly number "work" by stretching the loan term — that doesn't mean you're getting a good deal on the car itself.
- Watch for manufacturer incentives. Promotional rates as low as 0%–2.9% do occasionally appear directly from manufacturers, especially on new models needing to move off the lot.
- Consider a shorter term. Longer loans lower the monthly payment but meaningfully increase total interest paid — and increase the odds of owing more than the car is worth.
Two specifics worth checking before you buy: our tire size calculator compares fitments and the speedometer error between them, and the engine horsepower calculator estimates power from quarter-mile figures.
Frequently Asked Questions
Should I put taxes and fees on the loan or pay them upfront?
Paying upfront keeps your loan balance (and total interest) lower, but rolling them into the loan can make sense if cash on hand is tight — just know you'll pay interest on that tax and fee amount for the life of the loan.
What if I still owe money on my trade-in?
Enter the amount you still owe in "Amount Owed on Trade-in" — the calculator subtracts it from your trade-in's value to get your true net trade equity (which could be negative if you're "underwater" on the old loan, increasing the amount you need to finance).
Why does the same car cost different amounts of tax in different states?
Tax rates themselves vary by state and even by county/city, and states differ on whether they reduce the taxable amount for a trade-in — both factors this calculator accounts for based on your selected state.
Is a longer loan term ever a good idea?
It can lower your monthly payment enough to fit your budget, but it also means paying substantially more in total interest and staying "underwater" (owing more than the car's worth) for longer, since cars depreciate faster than a long loan pays down principal.
How is the reverse "Monthly Payment" tab different?
Instead of starting with a vehicle price and calculating your payment, this tab starts with the monthly payment you can afford and works backward to tell you the maximum vehicle price that keeps you at or under that number, given your other loan terms.
This calculator is provided for educational and estimation purposes only and does not constitute financial advice. Consult a lender or financial advisor for terms specific to your situation.