Car Loan Calculator

What this calculator does

A car’s sticker price is only the opening bid. Between sales tax, your down payment, and a trade-in, the amount you actually finance can differ from the sticker by thousands — and the loan’s rate and term then determine what the car truly costs you. This calculator assembles all of it: it computes sales tax with correct trade-in treatment, derives the amount financed, produces the monthly payment, and totals the complete cost of ownership on the price side (vehicle, tax, and interest).

How the math works

The calculation runs in three steps.

1. Sales tax. In most states, tax applies to the price minus your trade-in:

tax = (price − trade-in) × tax rate

2. Amount financed. Everything you’re borrowing after cash and trade-in are applied, with tax rolled in:

financed = price + tax − down payment − trade-in

3. Monthly payment. The standard amortization formula, identical to a mortgage:

M = P · r(1 + r)ⁿ / ((1 + r)ⁿ − 1)

with P the amount financed, r the monthly rate, and n the number of months.

A worked example

A $32,000 car with $4,000 down, no trade-in, 7% sales tax, financed at 6.9% for 5 years:

• Sales tax: 32,000 × 7% = $2,240
• Amount financed: 32,000 + 2,240 − 4,000 = $30,240
• Monthly payment: r = 0.069 ÷ 12 = 0.00575, n = 60 → $597.34
• Total interest: 597.34 × 60 − 30,240 ≈ $5,600
• Total cost of the car: 4,000 down + 35,840 in payments ≈ $39,840

That last number is the one dealers never volunteer: the $32,000 car costs nearly $40,000 once tax and financing are included. Add a $6,000 trade-in to the same deal and the tax drops to $1,820 (tax on $26,000), the financed amount falls to $23,820, and the payment lands near $470.

Common mistakes

1. Letting the dealer quote payment before price. A monthly number can be “made to work” by stretching the term regardless of the price — always fix the out-the-door price first, then compare financing.
2. Taxing the full price when trade-in is deductible. Most states tax price minus trade-in; assuming otherwise overstates the loan by hundreds of dollars of phantom tax.
3. Ignoring negative equity on a trade-in. Rolling what you still owe on the old car into the new loan means financing a car you no longer have — run the inflated amount financed here before agreeing.
4. Stretching the term to shrink the payment. A 72–84 month term lowers the monthly number but usually means owing more than the car is worth for years — check the total-cost figure, not just the payment.

Practical tips

1. Negotiate the out-the-door price first, financing second. Dealers can make a “great” monthly payment out of a bad price by stretching the term. Fix the price, then compare financing offers on that fixed number.
2. Get pre-approved before you shop. A credit union or bank pre-approval gives you a real benchmark rate; the dealer then has to beat it rather than anchor you.
3. Watch the 20/4/10 rule of thumb. A common budgeting guideline: 20% down, no more than a 4-year term, and total vehicle expenses (payment plus insurance and fuel) under 10–15% of take-home pay. It’s conservative on purpose — cars lose value fast.
4. Beware negative equity rollovers. If you owe more on your current car than its trade-in value, some dealers offer to roll the shortfall into the new loan. That means financing a car you no longer own — run the numbers here with the inflated amount financed before agreeing.

The trade-in tax advantage, quantified

Because most states tax only the difference between price and trade-in, trading in a $10,000 car doesn’t just reduce your loan by $10,000 — at a 7% tax rate it also saves $700 in tax. A private sale usually fetches more than a dealer’s trade-in offer, but the gap has to exceed the tax savings (plus your time and risk) to be worth it. This calculator lets you compare both scenarios in seconds: run it once with the trade-in, once without, and weigh the payments against what a private buyer would realistically pay.