How your car payment is calculated
Your monthly payment depends on how much you finance, your interest rate, and how long the loan lasts. The amount you finance isn't just the sticker price — it's the price plus sales tax and fees, minus your down payment and net trade-in. This calculator walks through each of those pieces so you can see exactly where your payment comes from.
Here is the full flow the tool uses:
- Net trade-in = trade-in value − what you still owe on it.
- Taxable amount = price, or price − trade-in if your state gives that credit.
- Sales tax = taxable amount × your state tax rate.
- Amount financed = price + tax + fees − down payment − net trade-in.
- Monthly payment =
L × r / (1 − (1 + r)^−n), where L is the amount financed, r is the APR ÷ 12 and n is the number of months.
The payment formula spreads the loan into equal monthly payments. Early payments are mostly interest because the balance is highest; later payments are mostly principal. Total interest is simply all your payments added up, minus the amount you borrowed.
A worked example, step by step
Say you're buying a $35,000 car with $3,000 down. The dealer offers $8,000 for your current car, which still has $2,000 left on its loan. Your state charges 6.5% sales tax and gives a trade-in tax credit, fees are $600, and your rate is 7.2% APR for 60 months.
- Net trade-in: $8,000 − $2,000 = $6,000.
- Taxable amount: $35,000 − $8,000 = $27,000, so sales tax is $1,755.
- Amount financed: $35,000 + $1,755 + $600 − $3,000 − $6,000 = $28,355.
- Monthly payment at 7.2% for 60 months: $564.14, with $5,494 of total interest.
Counting your down payment, your trade-in equity and all 60 payments, the car costs you about $42,849 in total.
How the loan term changes the cost
Same car, same $28,355 financed, same 7.2% rate — only the length changes:
| Term | Monthly payment | Total interest |
|---|---|---|
| 36 months | $878 | $3,257 |
| 48 months | $682 | $4,363 |
| 60 months | $564 | $5,494 |
| 72 months | $486 | $6,648 |
| 84 months | $431 | $7,826 |
Stretching from 36 to 84 months drops the payment by about $447 a month but adds about $4,569 in interest. In real life, lenders often charge a higher rate on longer terms, so the gap is usually bigger than this table shows.
Why the trade-in tax credit matters
In states that tax the price after trade-in, a valuable trade can cut your tax bill noticeably. In the example above, sales tax is $1,755 with the credit. Without it, tax is charged on the full $35,000 — $2,275 — which raises the amount financed to $28,875 and the payment to $574.49. Check your state's rule and set the toggle in the calculator to match.
Negative equity: the expensive mistake
If you owe more on your trade-in than it's worth, the shortfall is added to the new loan. Change the example so you still owe $11,000 on the $8,000 trade-in: the net trade-in becomes -$3,000, the amount financed jumps to $37,355, and the payment rises to about $743 a month with $7,237 of interest. A 2024 CFPB study of auto lending data found that loans with rolled-in negative equity had larger amounts, higher payments and higher interest rates, and these borrowers were more likely to have their account sent to repossession within two years. If you can, keep driving the old car a little longer or pay the gap down before you trade.
Paying a car loan off early
Most auto loans charge interest on the balance you still owe, so any extra you send to principal cuts future interest the same way it does on a mortgage. Before you prepay, read the contract for a prepayment penalty and ask the lender to apply the extra to principal rather than to your next payment. Because interest is front-loaded, extra payments in the first year or two save the most; near the end of the loan, most of each payment is already principal.
Common mistakes
- Shopping by monthly payment alone — a longer term can hide a higher price or rate.
- Forgetting tax and fees, which can add a few thousand dollars to the amount financed.
- Rolling add-ons (extended warranties, service plans) into the loan without pricing them separately.
- Entering the trade-in's retail value instead of the dealer's actual offer.
When this calculator doesn't fit
It assumes a simple-interest loan with equal monthly payments and a fixed rate. It doesn't model leases, balloon loans, manufacturer rebates applied before or after tax (rules vary by state), or local taxes added on top of the state rate. For those, use the dealer's itemized buyer's order and enter the final numbers here as a check.
Tips before you finance a car
- Get pre-approved by your bank or credit union so you know your real rate.
- Negotiate the vehicle price separately from the monthly payment.
- Put down enough to avoid being underwater as the car depreciates.
- Compare the total interest, not just the monthly payment, across loan terms.
- Check what the payment does to your budget with the paycheck calculator, and if you're carrying other debts, see how the new loan fits with the debt payoff calculator.