How to calculate your car loan payment
Short answer
A car loan payment uses the same amortization formula as a mortgage: M = P[r(1+r)^n] / [(1+r)^n - 1], where P is the amount financed, r is your monthly interest rate, and n is the number of payments. On a $28,000 car with $3,000 down (so $25,000 financed) at 6.9% APR over 60 months, the payment comes to about $493.85 a month, with roughly $4,631 in total interest. Stretch the same loan to 72 months and the payment drops to about $425 but total interest climbs to about $5,602. These are estimates; your actual rate and payment depend on your lender and credit.
Car loan payment formula
M = P[r(1+r)^n] / [(1+r)^n - 1]
- •M is the monthly payment
- •P is the amount financed: vehicle price + sales tax - down payment - trade-in value
- •r is the monthly interest rate: APR / 12, written as a decimal
- •n is the number of monthly payments, the loan term in months
The three numbers that set your payment
A car loan is an amortizing loan, the same math a mortgage uses, just over a shorter term. Three inputs decide your monthly payment: how much you actually finance, your interest rate, and how many months you're paying it back. Get any one of those wrong in your head and your payment estimate will be off.
The amount financed is not the sticker price. It's the vehicle price, plus sales tax if your state taxes the purchase, minus your down payment and any trade-in value. Your APR gets converted to a monthly rate by dividing by 12, and the term is simply the number of monthly payments, typically 36 to 72 months, sometimes longer.
The amortization formula
Car loans use the identical formula lenders use for mortgages: M = P[r(1+r)^n] / [(1+r)^n - 1]. It looks intimidating, but each piece is simple, and it's the reason a loan calculator can spit out an exact payment instead of a rough guess.
Worked example: $28,000 car, $3,000 down, 6.9% APR, 60 months
Say you're buying a $28,000 car and putting $3,000 down, with no trade-in and no tax rolled in for now. That leaves P = $25,000 financed at 6.9% APR over a 60-month term.
The monthly rate is r = 0.069 / 12 = 0.00575. Raise (1 + 0.00575) to the 60th power and you get about 1.41060. Plug that into the formula: M = 25,000 x 0.00575 x 1.41060 / (1.41060 - 1) = 202.77 / 0.41060, which works out to about $493.85 a month.
Multiply that by 60 payments and you pay about $29,631 total over the life of the loan, meaning about $4,631 of that is interest on top of the $25,000 you borrowed. That interest figure, not just the monthly number, is the real cost of the loan.
Trade-in value and rolling sales tax into the loan
A trade-in works exactly like extra down payment: its value (what the dealer credits you, not what you think it's worth) comes off the price before the loan amount is calculated. If you still owe money on the car you're trading in, only the equity, trade value minus your remaining loan balance, reduces what you finance.
Sales tax is different. Most states tax the vehicle purchase, and many buyers roll that tax into the loan rather than paying it upfront. On the $28,000 car at a 6% rate, that's $1,680 in tax. Adding it to the $25,000 financed brings the loan to $26,680, which raises the same 60-month payment from $493.85 to about $527.04, roughly $33 more a month for the life of the loan. Rolling in tax is convenient, but it means financing (and paying interest on) money that never went toward the car itself.
Term length: lower payment now, more interest overall
The loan term is the lever that trades a smaller monthly payment for a bigger total cost, and it's easy to underestimate by how much. Take the same $25,000 loan at 6.9% APR and stretch it from 60 to 72 months.
At 72 months, the payment drops from $493.85 to about $425.03, a savings of roughly $69 a month. But total interest rises from about $4,631 to about $5,602, an extra $971 for the privilege of a smaller payment. The longer the term, the more months your balance sits accruing interest before it's paid down, so a lower payment on paper is rarely a lower total cost.
Being underwater: when you owe more than the car is worth
Cars lose value the moment they're driven off the lot, and fast in the first couple of years. Negative equity, being 'underwater,' means your remaining loan balance is higher than what the car would actually sell for. It's common with small or no down payments, long terms, and financed tax and fees, because all of those keep the loan balance high while the car's value keeps dropping.
The danger shows up if you total the car, sell it, or trade it in early: insurance typically pays out the car's market value, not your loan balance, leaving you to cover the gap out of pocket. Trading in an underwater car rolls that negative equity into the new loan, so you start the next car already owing more than it's worth, a cycle that gets harder to escape each time.
Common mistakes
Most car-loan regret traces back to a handful of avoidable habits. These are worth checking against before you sign.
- Shopping the payment instead of the total cost: a dealer can hit almost any monthly number you ask for by stretching the term, but that number alone hides how much interest you're actually agreeing to pay.
- Ignoring the term's effect on total interest: as shown above, 12 extra months on a $25,000 loan added about $971 in interest for only $69 a month in relief.
- Ignoring your trade-in's payoff balance: if you owe more on your trade-in than it's worth, that gap gets added to your new loan, not forgiven, so you finance the old car's debt along with the new one.
- Skipping the amount-financed math: buyers who focus only on the sticker price forget that tax, fees, and a rolled-in negative trade-in can push the real loan amount thousands above the price they negotiated.
- Not shopping the rate separately from the payment: a dealer-arranged loan and your own bank or credit union can quote very different APRs for the same amount and term, and even a point or two changes the total interest meaningfully.
Working backward from the payment you can handle
Most shoppers start with a monthly number, not a price. The payment formula can be turned around: amount financed = payment x [(1+r)^n - 1] / [r(1+r)^n]. Say you want $450 a month at 7% APR for 60 months. The monthly rate is 0.07 / 12, and the formula gives $22,725.90 of financing. If you have $3,000 for a down payment and your state charges 6% sales tax, that supports a vehicle price of about $24,270, because 24,269.71 x 1.06 - 3,000 returns the same financed amount.
Now see what happens when the term stretches. At the same $450 and 7%, a 72-month loan supports $26,394.50 of financing, which is $3,668.60 more car for the same payment. The catch is interest: that loan costs $6,005.50 in interest, against $4,274.10 on the 60-month version. A longer term lets you shop higher, but you pay for it twice, in interest and in the months you may owe more than the car is worth.
How much the rate itself moves the payment
Interest rate quotes vary by lender, credit profile, the age of the vehicle, and the term, so it helps to feel the sensitivity. Hold the loan at $25,000 and 60 months, and change only the APR. At 3.9% the payment is $459.29 and total interest $2,557.14. At 5.9% it is $482.16 and $3,929.51. At 6.9% it is $493.85 and $4,631.08. At 9.9% it is $529.95 and $6,796.81, and at 14.9% it is $593.44 and $10,606.21.
Read the spread, not just the endpoints. Moving from 3.9% to 14.9% adds $134.15 a month and $8,049.07 in interest on the same car. Even a three point step, from 6.9% to 9.9%, adds $36.10 a month and $2,165.73 over the loan. That is why comparing offers by APR matters more than comparing monthly payments that were built on different terms or amounts. The rate you are quoted on a given day is a lender decision, and it may differ from the rate in an advertisement.
Zero percent financing versus a cash rebate
Manufacturers often offer a choice between promotional financing and a rebate, and the better deal depends on your alternative rate. Take a $30,000 car with no tax or down payment to keep the math clean. Option A is 0% for 60 months, which is exactly $500 a month and $30,000 in total. Option B takes a $3,000 rebate, finances $27,000 at a bank rate of 6.9% over 60 months, and costs $533.36 a month, or $32,001.56 in total.
In that case 0% saves $2,001.56. The rebate only wins if your outside rate is low enough: the break-even is about 4.23% on those terms. At 4.9% the bank loan costs $508.29 a month and $30,497.23 in total, so 0% still comes out ahead. Also check the fine print, because 0% offers are normally limited to specific models, shorter terms, and well-qualified buyers, and the rules for rebates, including how sales tax treats them, vary by manufacturer and state.
Rolling negative equity into the next loan
If you owe more on your trade than it is worth, that gap comes along with you. Suppose you owe $14,000 on a car valued at $11,000, leaving $3,000 of negative equity. You buy the $28,000 car with 6% tax and no down payment at 6.9% over 60 months. Without the old debt you would finance $29,680 and pay $586.30 a month. With it you finance $32,680 and pay $645.56, which is $59.26 more every month.
Over the loan, that extra $3,000 also generates $555.73 of interest, and you begin the new loan even deeper underwater. The Auto Loan Calculator has no field for negative equity, so model it by lowering the trade-in value or raising the price by the amount you owe. Many lenders also cap how much they will finance relative to the car's value. Gap coverage, offered by dealers and some insurers, pays the difference between the car's value and your loan balance if it is totaled, so compare its price against how long you will be upside down.
A budget check using the 20/4/10 guideline
A well-known conservative rule of thumb for car buying is 20/4/10: put down at least 20 percent, finance for no more than four years, and keep total vehicle costs at or under 10 percent of gross income. It is a guideline, not a law, and many buyers will not hit all three, but it is a useful ceiling check. With a $6,000 gross monthly income, 10 percent is $600 for everything, not just the loan.
Subtract the rest. Suppose insurance is $150, fuel $120, and maintenance $50, which are assumptions you should replace with your own. That leaves $280 for the payment. At 7% over 48 months, $280 supports about $11,692.86 of financing, and if that is 80 percent of the price, the car costs around $14,600. The result is a modest used vehicle, which is exactly the point: it shows how quickly a full-feature purchase outruns a conservative budget.
Shopping the loan itself
Treat the financing as a separate purchase from the car. Getting a preapproval from a bank, credit union, or online lender before you visit a dealer gives you a rate to measure against, and dealers can sometimes beat it. Applications for the same type of loan made within a short window, often around 14 days, are generally treated as one inquiry by scoring models, although the exact window varies by model. Ask whether the loan is simple interest, whether there is a prepayment penalty, and what the APR is after all fees.
Add-ons deserve their own math. A $500 item financed into the $25,000 loan at 6.9% over 60 months raises the payment by about $9.88 and costs roughly $593 across the term. Paying extra helps too: adding $75 a month to the $25,000 loan ends it in 51 months and cuts interest from $4,631.08 to $3,900.70, a savings of $730.38, provided the lender applies the extra to principal.
Frequently Asked Questions
How much of my car payment goes to interest at first?+
More than you'd guess from balance divided by payment. In the worked example, the first month's interest on $25,000 at 6.9% APR is 0.00575 x 25,000, about $144. Out of a $493.85 payment, roughly $350 reduces the balance and the rest covers interest. As the balance falls each month, less of the payment goes to interest and more goes to principal, but the early months are the slowest for building equity.
Is a longer loan term ever the right call?+
Sometimes, if the lower payment genuinely fits your budget better and you understand the trade-off going in. The math is clear: a 72-month term on the $25,000 example costs about $971 more in total interest than 60 months, in exchange for about $69 a month in relief. A longer term also means more months of owing more than the car is worth, since depreciation runs faster than a stretched-out loan pays down principal.
Should I roll sales tax and fees into my loan?+
You can, and many buyers do for convenience, but it isn't free. Rolling $1,680 in tax into the $25,000 example loan raised the monthly payment by about $33 and means you're financing, and paying interest on, money that never went toward the car. If you can pay tax and fees in cash instead, you finance a smaller amount and pay less interest overall.
What does it mean to be underwater on a car loan, and how do I avoid it?+
Being underwater means you owe more on the loan than the car is currently worth, which happens fastest with small down payments, long terms, and financed taxes or fees. A larger down payment (even 10-20%) and a shorter term both keep your loan balance closer to the car's actual value as it depreciates. Before trading in a car you might still owe money on, check its current market value against your payoff balance so you know whether you're carrying negative equity into the next loan.
Does refinancing a car loan make sense?+
It can if your rate has dropped or your credit has improved. Suppose the $25,000, 6.9%, 72-month loan has had 12 payments made, leaving a $21,515.87 balance. Refinancing that to 4.9% over 60 months lowers the payment from $425.03 to $405.05 and saves about $1,199 in interest, before any fees. Check for title and lender fees, and avoid restarting a longer term.
Is a bigger down payment always better?+
Usually, because it reduces the amount financed, the interest, and the risk of owing more than the car is worth. It also may help you qualify for a better rate with some lenders. The trade-off is liquidity: do not drain your emergency savings to avoid a loan. One approach is to keep a full emergency fund first and put the remainder toward the down payment.
How does my credit score affect the payment?+
Lenders price risk, so borrowers with stronger credit generally receive lower APRs. As shown on a $25,000, 60-month loan, moving from a 6.9% to a 9.9% rate adds about $36 a month and over $2,100 in interest. Because each lender sets its own tiers, check your credit reports for errors before applying and compare several quotes rather than relying on a general estimate.
Should I pay for the extended warranty with the loan?+
Be cautious. Financing it means you pay interest on it, and the price is often negotiable. Compare what the plan covers, the deductible, who backs it, and whether you can cancel for a prorated refund. A $500 add-on financed at 6.9% over 60 months costs roughly $593 in total. If you want coverage, get quotes from other providers before you sign anything.
Sources & further reading
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