Say you're buying a $32,000 vehicle. You put $4,000 down and trade in your old car for $3,000. Your state charges 7% sales tax, and most states tax the vehicle price net of the trade-in — so the taxable amount here is $32,000 − $3,000 = $29,000, and the tax owed is $29,000 × 7% = $2,030.
Rolling that tax into the loan (the norm for most financed purchases), the amount you actually finance is: $32,000 (price) − $4,000 (down) − $3,000 (trade-in) + $2,030 (tax) = $27,030. That's your loan amount, P, in the formula below.
Running the Amortization Formula
With a 6.5% APR over 60 months, the monthly interest rate is r = 0.065 ÷ 12 = 0.0054167, and n = 60 payments. Plugging into the standard installment-loan formula M = P × r(1+r)n ÷ [(1+r)n − 1]:
M = $27,030 × 0.0054167 × (1.0054167)60 ÷ [(1.0054167)60 − 1] = $528.87 per month.
Over 60 months that's a total of $528.87 × 60 = $31,732.38 paid, against a $27,030 loan amount — meaning $4,702.38 goes to interest alone. Change any input above to this exact scenario (price 32000, down 4000, trade-in 3000, tax 7, APR 6.5, term 60) and you'll get these same figures back.
Why Sales Tax Matters More Than People Think
Buyers often mentally budget for "price minus down payment minus trade-in" and forget tax entirely. In the example above, tax alone added over $2,000 to the financed amount, which — spread across 60 months at 6.5% — adds roughly $32 to every single monthly payment. Because trade-in tax treatment genuinely varies by state (some tax the full price, some don't offer a trade-in credit at all, and a few have no sales tax on vehicles whatsoever), always verify your specific state's rule before treating this number as final. This calculator assumes the common "tax on net price after trade-in" model.
Auto Loan Calculator vs. a General Loan Amortization Tool vs. APR Calculator
This calculator exists because car purchases have inputs a generic loan calculator doesn't handle well: a vehicle price, a trade-in that reduces both the loan and the taxable amount, and a sales tax rate that varies by state. If you already know your exact loan amount and just want a plain month-by-month principal/interest schedule for any type of installment loan, the Loan Amortization Calculator gives you that ledger directly. And if a dealer or lender is quoting you fees on top of a rate and you want to know the true annualized cost of borrowing, the APR Calculator is the better tool for translating fees-plus-rate into one comparable number.
Tips for a Lower Payment
- Shop your own financing (credit union or bank pre-approval) before you're in the dealership finance office, so you have a rate to compare against.
- A bigger down payment reduces both your loan amount and — often — the interest rate lenders will offer, since it lowers their risk.
- Shorter terms cost more per month but dramatically less in total interest; use this calculator to compare 48, 60, and 72 months side by side before committing.
Further reading: Investopedia — Car Loan Calculator and Wikipedia — Amortization Calculator.
Disclaimer: This calculator and the notes above are for educational purposes only and should not be considered financial advice. Sales tax treatment of trade-ins varies by state, and actual loan terms, fees, and taxes depend on your lender and location. Always verify figures with your dealer, lender, and state DMV before making borrowing decisions.