Auto Loan Calculator

How your car payment is calculated

An auto loan payment starts with the same amortization formula used for any installment loan, but the amount actually being financed first needs to account for sales tax added and any down payment or trade-in value subtracted.

Worked example: a $35,000 vehicle with 6.5% sales tax ($2,275), a $5,000 down payment, and no trade-in leaves a loan amount of $32,275. At 7.0% over 60 months, this produces a monthly payment of roughly $639, with total interest of about $6,070 over the life of the loan.

Sales tax is easy to overlook when mentally budgeting for a car purchase, since dealers often quote a price before tax — but the tax amount gets rolled directly into the amount financed in most states, meaning it accrues interest right alongside the vehicle price itself for the entire loan term.

This same formula and structure applies whether the vehicle is new or used, and regardless of loan source (dealer, bank, or credit union) — the underlying math of amortization doesn’t change based on where the loan comes from. What differs between financing sources is the rate offered and any fees involved, not the calculation method itself.

How sales tax affects your loan

Sales tax rates on vehicle purchases vary significantly by state — some states charge a standard sales tax rate identical to other retail purchases, some states have specific (often lower) rates for vehicles, and a handful of states charge no sales tax on vehicle purchases at all. Because this tax typically gets financed as part of the loan rather than paid entirely upfront, it doesn’t just add to the purchase price — it adds to the amount actually earning interest over the loan term.

Trade-in value in many states reduces the taxable amount, not just the amount financed — meaning a trade-in can lower both the sales tax owed and the loan amount simultaneously, a meaningfully larger benefit than a cash rebate of the identical dollar value would provide. This specific tax treatment varies by state, so confirming how a specific state handles trade-in tax credit is worth doing before assuming a trade-in’s full value translates into tax savings.

A handful of states charge no sales tax on vehicle purchases at all, and a few others cap the taxable amount or apply a flat fee instead of a percentage rate. Because this variation is entirely state-specific, using the actual rate for the state where a vehicle will be registered — rather than a generic national assumption — produces a meaningfully more accurate total cost estimate, particularly for buyers purchasing across state lines or relocating shortly after a purchase.

Choosing the right loan term

A 48–60 month term is generally considered the sweet spot for balancing an affordable monthly payment against reasonable total interest cost. Stretching to 72 or 84 months lowers the payment further, but meaningfully raises total interest paid and increases the risk of being “underwater” — owing more on the loan than the car is currently worth — since vehicles depreciate quickly, especially in the first few years, while a longer loan term pays down the balance more slowly.

Longer loan terms have become considerably more common in recent years as vehicle prices have risen, with 72- and even 84-month terms now a routine offering rather than a rare exception. This shift makes it more important than ever to look past the monthly payment figure alone — a longer term can make an otherwise expensive vehicle appear deceptively affordable on a month-to-month basis, while the total cost and depreciation-risk tradeoffs remain very real regardless of how manageable the payment looks in isolation.

Being underwater matters most at the exact moments a vehicle might need to be sold, traded in, or is declared a total loss after an accident. In any of these situations, owing more than the vehicle is worth means the difference has to be paid out of pocket or rolled into a new loan — a genuinely disadvantageous position that a shorter loan term, or a larger down payment, directly reduces the risk of.

Gap insurance addresses this specific risk directly for drivers who do choose a longer term or a smaller down payment — it covers the difference between a vehicle’s actual cash value and the remaining loan balance if the car is totaled or stolen, precisely the scenario where being underwater becomes a real financial problem rather than an abstract concern. This coverage is inexpensive relative to the protection it provides, and it’s worth considering specifically for any loan structure where the underwater period is likely to be both deep and prolonged.

New vs. used car financing

New car loans typically carry meaningfully lower interest rates than used car loans, largely because new vehicles have more predictable value and depreciation patterns, making them a lower-risk asset for a lender to finance. Used cars carry more uncertainty about condition and remaining useful life, which lenders price into a higher rate.

A higher rate on a used car doesn’t automatically mean a used car is the more expensive overall choice. Since used vehicles typically cost substantially less than a comparable new vehicle, the lower purchase price can more than offset the higher interest rate — the total cost of financing a used car is often still lower than financing a new one, even accounting for the rate difference. Running both scenarios through the same calculation is the clearest way to compare the actual total cost rather than assuming the lower advertised rate on new cars always wins.

Certified pre-owned (CPO) vehicles occupy a middle ground worth knowing about. These are used vehicles that have passed a manufacturer-specified inspection and typically come with an extended warranty, and some manufacturers offer CPO-specific financing rates that sit between standard new and used rates — narrowing the rate gap while still offering a lower purchase price than a comparable new vehicle. Checking whether CPO-specific financing is available is worth doing for any used-vehicle purchase from a franchised dealership.

Dealer financing vs. bank financing

Dealership financing offers genuine convenience — the loan is arranged as part of the same transaction as the vehicle purchase — and dealers sometimes offer manufacturer-subsidized promotional rates (including 0% APR offers on select new vehicles) that can beat any independently arranged financing. However, dealers can also mark up the interest rate on financing they arrange through a lending partner, since that markup is a source of dealer profit separate from the vehicle sale itself.

Getting pre-approved by a bank or credit union before visiting a dealership is a widely recommended strategy for exactly this reason — walking in with an already-secured financing offer provides a concrete baseline to compare the dealer’s financing against, and creates genuine negotiating leverage, since the dealer then has to beat (or match) an offer that’s already on the table rather than being the only financing option presented.

Credit unions in particular often offer meaningfully lower rates than large banks or dealer-arranged financing, reflecting their member-owned, not-for-profit structure. Membership eligibility requirements vary by credit union, but many have fairly broad eligibility criteria (based on location, employer, or association membership) that make joining accessible to a wide range of borrowers — worth checking before assuming credit union financing is out of reach.

Real-world applications

Deciding between a dealer’s promotional financing and an independent bank loan benefits directly from comparing the two using identical vehicle price, term, and tax assumptions — a 0% dealer promotion is often the better deal outright, but a marked-up dealer rate might lose to an independently secured loan once actually compared side by side.

Evaluating whether a trade-in is worth pursuing versus selling a vehicle privately benefits from factoring in the trade-in’s tax-reduction effect in states where it applies — even a trade-in offer below a vehicle’s private-sale value can sometimes net out favorably once the tax savings on the new purchase are factored in.

Setting a target loan term based on affordability without falling into the underwater-loan trap benefits from checking the total interest and vehicle-cost figures at multiple different term lengths side by side, rather than defaulting to whatever term produces the lowest monthly payment in isolation.

Comparing a 0% manufacturer promotional rate against taking a rebate with standard financing instead is a common decision point at new-car dealerships, since manufacturers often present these as alternative offers rather than allowing both simultaneously. Running the numbers on both paths — the 0% loan on the full price versus a lower price (after rebate) financed at the standard rate — makes clear which option actually produces the lower total cost for a specific vehicle price and loan term, since the better choice isn’t always the obvious one at first glance.

Common mistakes to avoid

• Overlooking sales tax when budgeting for a car purchase. Sales tax is typically rolled into the financed amount in most states, meaning it accrues interest for the full loan term right alongside the vehicle price.
• Choosing the longest available loan term purely to minimize the monthly payment. Longer terms increase total interest substantially and raise the risk of being underwater on the loan, especially in the first few years when depreciation is steepest.
• Assuming a used car’s higher interest rate makes it the more expensive overall option. The lower purchase price of a used vehicle often more than offsets a higher rate — comparing total cost, not just rate, gives the accurate picture.
• Accepting dealer financing without comparison-shopping first. Getting pre-approved by a bank or credit union before negotiating provides genuine leverage and a real baseline for comparison.
• Not accounting for how a trade-in reduces taxable amount in states where that applies. This can make a trade-in more valuable than its raw dollar amount alone would suggest.
• Treating advertised “average” rates as a personal guarantee. Actual rates depend heavily on credit score, lender, and loan term — the published averages are a useful benchmark, not a specific quote.
• Skipping gap insurance on a long-term or low-down-payment loan. This relatively inexpensive coverage specifically protects against the underwater-loan scenario if a financed vehicle is totaled or stolen — worth considering whenever the loan structure makes an extended underwater period likely.
• Negotiating the monthly payment instead of the vehicle price and interest rate separately. A dealer can hit almost any target monthly payment simply by adjusting the loan term — negotiating price and rate independently, then checking the resulting payment, prevents a seemingly attractive payment from masking an unfavorable price or an extended term.