Auto Loan Calculator Guide
Car payments go wrong before the interest rate ever enters the picture: the mistakes live in the amount financed. Price, tax, down payment, and trade-in interact in ways that dealer paperwork doesn't always make obvious — this guide covers assembling that number correctly and reading offers against it.
The Amount Financed, Assembled
| Vehicle price | 30,000 |
|---|---|
| + Sales tax (7% on price − trade-in) | 1,750 |
| − Down payment | 3,000 |
| − Trade-in | 5,000 |
| = Amount financed | 23,750 |
Note the tax line: in most US states, tax applies to price minus trade-in — the worked example saves 350 versus taxing the full price. Our auto loan calculator handles both rules with a toggle; outside the US, taxes are usually in the sticker price, so leave the rate at zero. From here the payment is ordinary amortization — the same engine explained in the amortization schedule guide.
Reading a Dealer Offer Against the Estimate
Rebuild the dealer's number in the calculator with your figures. Two gaps are diagnostic. If their amount financed exceeds yours, the difference is fees and add-ons — each one negotiable. If the amounts match but the payment differs, the rate differs from what was advertised; ask for the contract APR. The calculator's "total cost of the vehicle" line converts any offer into one comparable number.
Structuring the Loan Well
Three structural choices dominate the outcome. Down payment: 20% is the classic depreciation buffer — the car down payment guide covers when less is defensible. Term: shortest payment you can hold; past 72 months negative equity becomes the norm. Total budget: the 20/4/10 affordability rule ties all three into one sanity check before you shop.
Frequently Asked Questions
Why does the dealer's payment differ from my estimate?
Dealer quotes fold in documentation fees, add-ons, extended warranties, and sometimes a marked-up rate.
Rebuild their number in the calculator: if the amount financed is higher than price plus tax minus your down
and trade-in, ask what filled the gap.
Should I finance the sales tax?
Rolling tax into the loan is common but means paying interest on the tax. Paying tax and fees in cash keeps
the loan at or below the vehicle's value from day one.
What term should I choose for a car loan?
The shortest term whose payment fits your budget — commonly 48 to 60 months. Beyond 72 months, interest
accumulates while the car depreciates fastest, and negative equity becomes likely.