What a car payment really includes
Dealers quote a monthly payment; this calculator rebuilds it from the pieces so you can negotiate the pieces instead. The amount you actually finance is: vehicle price − down payment − trade-in value + sales tax. Most states charge sales tax only on the difference between the new car's price and your trade-in — a detail worth hundreds of dollars that rarely appears in ads.
The auto loan formula
The payment itself is the standard amortization formula used by the Loan Calculator. Nothing about a car loan's math is special — which is exactly why extending the term to lower the payment is so expensive: the same interest engine runs for more months.
Term length: the quiet budget-killer
Loans of 72 and 84 months are now common, pushed by rising car prices. On $28,000 financed at 7%: 60 months costs $554.43/month and $5,266.01 of interest; 84 months drops the payment to $422.60 but interest climbs to $7,497.98 — and the car may be worth less than the balance for most of the loan ("underwater"). If a payment only fits at 84 months, the car — not the term — is the problem.
Before you sign
- Get a pre-approval from your bank or credit union first; dealer financing must then beat a real number.
- Negotiate the vehicle price, trade-in value and financing as three separate deals — mixing them hides costs.
- Watch for add-ons rolled into the balance (extended warranties, GAP, protection packages): each raises the financed amount and the interest on it.
Worked example
You buy a $35,000 car with $5,000 down and a $3,000 trade-in, in a state with 6% sales tax. Tax applies to $32,000 (price minus trade-in) = $1,920, so you finance $35,000 − $5,000 − $3,000 + $1,920 = $28,920. At 7% for 60 months the payment is about $572.65 and total interest about $5,439.04 — numbers a dealer quote should match closely.
How to use this calculator
- Enter the vehicle's negotiated price — not the sticker, if you have a real quote.
- Enter your cash down payment and your trade-in's value as separate lines.
- Enter your state sales tax rate; the trade-in tax credit is applied automatically.
- Enter the APR and term from your best financing offer.
- Compare the total interest across terms before agreeing to a longer loan for a lower payment.
Should you put more money down?
A larger down payment cuts interest and keeps you right-side-up on the loan, but it also drains cash you might need for emergencies. A common compromise: put down enough that the financed amount is comfortably below the car's expected value after a year, and keep the rest liquid.