Auto Loan

How Auto Loan Payments Are Calculated

An auto loan payment is calculated on the vehicle price minus any down payment and trade-in value, plus sales tax, financed at the loan’s rate and term. Enter those figures and this calculator estimates your monthly payment — built the way an actual car purchase works, not just a generic loan amount you’d have to compute yourself first.

Your down payment and trade-in value both reduce the amount you finance, but only your trade-in value reduces the amount sales tax is calculated on — a down payment is paid from already-taxed money, while a trade-in is treated as part of the transaction itself. This trade-in tax credit is how most US states handle it, but not all states give it, and the rule can change — check your own state’s rule and uncheck “My state taxes only the price minus the trade-in value” if it doesn’t apply to you, and the calculator taxes the full vehicle price instead.

If you still owe money on the vehicle you’re trading in, enter that under “Amount owed on trade-in.” Most of the time, the trade-in’s value simply reduces what you finance — but if the amount owed is more than the trade-in is worth (a common situation called being “underwater” or having negative equity), that difference doesn’t disappear — it gets rolled into your new loan instead, increasing the amount financed. Under “Add cash incentives, fees” you can also enter any manufacturer rebates (which reduce the amount financed, same as a down payment) and title/ registration/other fees, plus choose whether to roll those fees into the loan or pay them upfront.

The Formula

Taxable amount and financed amount:

$$\vF{\text{Sales Tax}} = (\vA{\text{Vehicle Price}} - \vB{\text{Trade-In Value}}) \times \text{Tax Rate}$$ $$\vE{\text{Net Trade-In Equity}} = \vB{\text{Trade-In Value}} - \vC{\text{Amount Owed on Trade-In}}$$ $$\text{Amount Financed} = \vA{\text{Vehicle Price}} + \vF{\text{Sales Tax}} + \text{Fees (if Rolled In)} - \vD{\text{Down Payment}} - \text{Cash Incentives} - \vE{\text{Net Trade-In Equity}}$$

Note that a negative net trade-in equity (an underwater trade-in) subtracts a negative number — which adds to, rather than reduces, the amount financed.

Monthly payment (the standard fixed-rate amortization formula):

$$M = P \times \frac{r(1+r)^{n}}{(1+r)^{n} - 1}$$

where $P$ is the amount financed, $r$ is the monthly interest rate (annual rate divided by 12), and $n$ is the number of monthly payments.

Worked Example

A $30,000 vehicle, with a $3,000 down payment, a $2,000 trade-in, 7% sales tax, a 6.5% APR, and a 60-month loan:

1. Sales tax: $(\vA{\$30,000} - \vB{\$2,000}) \times 7\% = \vF{\$1,960}$.
2. Amount financed: $\vA{\$30,000} + \vF{\$1,960} - \vD{\$3,000} - \vB{\$2,000} = \$26,960$.
3. Monthly payment: $527.50.
4. Total paid over 60 months: $527.50 × 60 = $31,650.20, so total interest is $31,650.20 − $26,960 = $4,690.20.

Key Factors to Consider

• A longer loan term lowers your monthly payment but usually increases total interest paid. Stretching the same amount financed over more months reduces each payment, but you pay interest for longer overall — compare total interest across term lengths, not just the monthly payment, before choosing a longer term purely to fit a budget.
• New vehicles typically qualify for lower rates than used vehicles. Lenders generally view new-vehicle loans as lower risk (a known, verifiable value with a full warranty) than used- vehicle loans, which is why advertised rates are often noticeably better for new cars.
• Gap insurance matters more with a small down payment or a rolled-in negative equity balance. A vehicle depreciates quickly, and a loan with little money down (or one that rolled in a previous loan’s negative equity) can owe more than the car is worth for a while — gap insurance covers that difference if the car is totaled before the loan catches up with its value.
• Pre-approval from a bank or credit union gives you real negotiating leverage at the dealer. Walking in with a pre-approved rate means you can compare the dealer’s financing offer directly against a real alternative, rather than negotiating price and financing terms at the same time with only the dealer’s own numbers to go on.

Common Mistakes

• Assuming trade-in tax credit rules are the same everywhere. Whether a trade-in reduces the taxable amount (and by how much) varies by state — check the per-state toggle above rather than assuming your state works like a neighboring one.
• Rolling negative equity into the new loan without realizing it. If a trade-in is still worth less than what’s owed on it, that shortfall gets added to the new loan’s amount financed — quietly increasing the loan size and total interest paid beyond the new vehicle’s own price.
• Only comparing the dealer’s financing offer. Dealer-arranged financing is convenient, but a pre-approved rate from a bank or credit union is often worth comparing before signing — the rate used here should be the one you’re actually being offered, not an assumed “typical” rate.

Useful to Know

Most U.S. auto loans use simple interest, calculated only on your current outstanding balance — not a fixed schedule set in stone at signing. That means extra payments (or paying a little early each month) reduce the principal balance sooner, which lowers the interest that accrues on every payment afterward, shrinking your total interest paid without needing to refinance. It also means paying a few days late doesn’t just cost you a fee in some cases — interest can keep accruing daily on the unpaid balance until your payment posts. Check your own loan agreement for whether extra payments are applied to principal by default or need to be specified — some lenders apply an overpayment to your next due date instead unless you tell them otherwise.