Auto Loan Calculator: Monthly Car Payment, Tax & Total Interest

Last updated: August 5, 2026 · Reviewed by the DoCalc team

What Is an Auto Loan Calculator?

A car payment is more than "price ÷ months" — sales tax gets financed along with the vehicle in most states, a down payment or trade-in reduces what you borrow, and interest accrues on the declining balance over the loan term. This calculator combines all four pieces into one accurate monthly payment, plus the total interest you'll pay over the life of the loan.

The Formula

Amount Financed = Price + (Price × Tax Rate) − Down Payment Monthly Payment = Amount Financed × [r(1+r)ⁿ] / [(1+r)ⁿ − 1]

Where r is the monthly interest rate (APR ÷ 12) and n is the loan term in months. This is the same standard loan amortization formula used for mortgages and personal loans — auto loans just add the sales-tax step before financing.

Worked Example

A $32,000 vehicle, 6.5% sales tax, $4,000 down, at 7.5% APR over 60 months:

Sales tax = $32,000 × 6.5% = $2,080 Amount financed = $32,000 + $2,080 − $4,000 = $30,080 Monthly payment ≈ $603 Total interest ≈ $6,084 over 60 months

Notice the tax adds real financed principal — skipping it in a back-of-envelope estimate is the most common reason a dealer's actual payment comes in higher than a quick mental calculation.

Loan Term: Shorter vs Longer

Term	Monthly payment	Total interest	Best for
36 months	Highest	Lowest	Minimizing total cost, building equity fast
48–60 months	Moderate	Moderate	The most common balance of payment vs. total cost
72+ months	Lowest	Highest	Maximizing monthly cash flow — at the cost of more interest and longer "underwater" risk

Pros and Cons of a Longer Term

Pros: lower monthly payment, more room in the monthly budget, easier qualification on a fixed income.

Cons: more total interest paid, slower equity buildup relative to the car's depreciation, longer period where you could owe more than the car is worth if you need to sell or trade in early.

Who Should Use This Calculator

Use it if you're shopping for a new or used car and want to see your real monthly payment — including tax — before you're sitting at the dealer's desk.

Adjust for it if your state taxes only the price minus trade-in (rather than the full price), or if the dealer is offering a promotional/subvented rate different from market APR — use the actual rate you're quoted for an accurate number.

Common Mistakes to Avoid

The most common mistake is comparing payments across different loan terms without noticing the total-interest tradeoff — a lower payment on a 72-month loan can cost thousands more overall than a 60-month loan at the same rate. A second mistake is forgetting sales tax entirely when estimating a payment, which understates the real number. A third is not shopping your own financing (bank or credit union) before accepting the dealer's rate, since dealer financing isn't always the cheapest option available to you.

Expert Recommendation

Get pre-approved by your own bank or credit union before visiting the dealer — it gives you a real rate to compare against (or negotiate with) the dealer's financing offer, and it removes the pressure of deciding on financing terms on the spot.

Frequently Asked Questions

Should sales tax be included in the loan amount?

In most states, yes — dealers typically roll sales tax into the amount financed rather than requiring it upfront, which is why this calculator adds tax to the vehicle price before subtracting your down payment and trade-in. A few states tax only the difference between the vehicle price and trade-in value (a "trade-in tax credit") — check your state's rule, since it changes the tax amount.

Does a longer loan term always mean a lower payment?

Yes, a longer term (say 72 months instead of 60) lowers the monthly payment, but it also means paying interest for longer and, on longer terms, risking being "underwater" — owing more than the car is worth — for a bigger chunk of the loan, since cars depreciate faster than a long loan pays down principal.

Is it better to put more money down?

A larger down payment reduces the amount financed, which lowers both the monthly payment and total interest paid — and it reduces the risk of owing more than the car is worth in the first year or two, when depreciation is steepest.

How does a trade-in affect my loan?

A trade-in reduces the amount you need to finance the same way a down payment does. Enter your trade-in's actual value (what the dealer credits you, not what you think it's worth) in the down payment field for an accurate estimate.

What's a typical auto loan interest rate?

Rates vary widely by credit score, loan term, and whether the vehicle is new or used — new-car loans typically carry lower rates than used-car loans, and borrowers with excellent credit typically qualify for meaningfully lower rates than the market average. Check current rates from your bank, credit union, or the dealer's financing before assuming a specific number.

Should I finance through the dealer or my bank?

It's worth comparing both. Dealers sometimes offer promotional rates (occasionally 0%) on select models, but your own bank or credit union may offer a better rate on your specific credit profile — getting pre-approved by your bank before visiting the dealer gives you a real number to negotiate against.

Does this calculator include registration and title fees?

No — this calculator covers vehicle price, sales tax, down payment/trade-in, and loan interest, which are the largest and most variable components of a car payment. Registration, title, and documentation fees vary by state and dealer and are typically smaller, one-time costs on top of what's shown here.

What loan term should I choose?

A common guideline is to choose the shortest term you can comfortably afford — 36 or 48 months keeps total interest low and avoids being underwater for long, while 60 or 72 months lowers the monthly payment at the cost of more total interest and slower equity building.

Conclusion

The real cost of a car loan is the combination of price, tax, term, and rate — not just the sticker price divided by a number of months. Run your specific numbers above before you're negotiating at the dealership, and compare at least two loan terms to see the real payment-vs-interest tradeoff.

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