How This Auto Loan Calculator Works
A car payment has three ingredients: how much you borrow, the interest rate, and how long you take to repay. This calculator starts from the sticker price, then applies the two things that change the borrowed amount in opposite directions — sales tax (added) and your down payment plus trade-in (subtracted) — and amortizes what's left over your chosen term.
The math is the standard loan amortization formula:
M = P × r(1+r)n / ((1+r)n − 1)
where
M = monthly payment, P = amount financed, r = monthly rate (APR ÷ 12), n = months.
Worked example: a $35,000 car with a $3,000 trade-in is taxed on $32,000 in most states. At 7% sales tax that's $2,240 of tax, so the total is $37,240. Subtract a $5,000 down payment and the $3,000 trade-in and you finance $29,240. At 7% APR over 60 months, the formula gives a monthly payment of about $579, total interest of roughly $5,500, and a true out-of-pocket cost (down payment + trade-in + all payments) of about $42,740.
Notice how the tax treatment matters: because most states tax only the price minus trade-in, a $3,000 trade-in saves you $210 in tax at 7% — a small but real discount on top of the trade-in's face value. The year-by-year schedule shows how each payment splits between principal and interest, which is useful if you plan to sell or refinance early: in the first two years, a big share of every payment is still interest.
Practical tips
A bank or credit union pre-approval gives you a rate to beat and turns you into a cash buyer at the negotiating table. Dealers can mark up financing — your pre-approval is your leverage.
Stretching a loan to 84 months can cut the monthly payment dramatically while adding thousands in interest — and you'll owe more than the car is worth for most of the loan. Aim for 60 months or less.
Cars depreciate fastest in year one. A solid down payment keeps you from going "underwater" (owing more than the car's value), which matters if the car is totaled or you need to sell.
Dealers love "what monthly payment works for you?" because they can hit any payment by extending the term. Settle the out-the-door price first, then talk financing.
Auto refinancing is quick and often free of fees. If your score has risen since purchase, a refinance can cut your rate — run the new numbers here first.