Estimate your monthly auto payment, total interest, and the true cost of a car. Adjust price, down payment, trade-in, APR, and loan term to find a number you can actually afford — and see what every dollar of interest would’ve been worth invested instead.
Enter the vehicle’s out-the-door price, your down payment, any trade-in value, the loan term in months, and your interest rate (APR). The calculator instantly shows your monthly payment, the total interest you’ll pay over the life of the loan, and the loan amount after tax and trade-in are applied. Sales tax is rolled into the financed amount — the standard treatment in most US states.
Auto loans use the standard amortization formula. Every monthly payment is split into two pieces: interest charged on the remaining balance, and principal that pays down the loan. Early in the loan, most of your payment is interest. Late in the loan, most of it is principal. The shape on the chart above is the same shape every borrower sees — the only thing that changes is the size.
P is the loan principal (vehicle price minus down payment and trade-in, plus financed sales tax). r is the monthly interest rate (APR ÷ 12 ÷ 100). n is the number of monthly payments (loan term in months). For a $30,000 loan at 7.5% APR over 60 months, the formula yields a monthly payment of roughly $601 and total interest of about $6,034.
The most-cited rule of thumb is the 20/4/10 rule:
If a car fails this test, it’s a stretch. That’s not a moral judgment — it’s a math one. Stretching on a depreciating asset quietly compounds against you for the next decade.
Every dollar you send to a lender is a dollar that didn’t compound for you. The interest line on the chart above isn’t just money lost — it’s money that, invested in a broad market index over the same timeframe, would have grown into a meaningfully larger number. That’s the “opportunity cost” the calculator surfaces in the green callout: today’s decision, multiplied across decades.
A car loan payment uses the standard amortization formula: M = P × (r(1+r)^n) ÷ ((1+r)^n − 1), where P is the loan amount (price minus down payment and trade-in, plus sales tax if financed), r is the monthly interest rate (APR ÷ 12 ÷ 100), and n is the number of monthly payments. The calculator above runs this formula in real time as you adjust the inputs.
As of early 2026, the average APR is roughly 7–8% for new cars and 8–10% for used cars. Borrowers with excellent credit (FICO 760+) can typically secure 5–6% on new vehicles. Anything above 11% on a new car loan is a red flag — either your credit needs work or the dealer is marking up the rate above what the lender approved.
A common rule is 20% down on a new car and at least 10% down on a used car. The reason: new cars depreciate roughly 20% in the first year, so a smaller down payment immediately puts you "underwater" — owing more than the car is worth. A larger down payment reduces interest over the life of the loan and protects you if the car is totaled or you need to sell early.
No. Longer loan terms (72 or 84 months) reduce the monthly payment but dramatically increase the total interest paid and keep you underwater on the loan for years. The 4-year (48-month) maximum is the most common guideline. If you cannot comfortably afford a car on a 48 or 60-month loan, the honest answer is the car is too expensive for your budget.
Credit score is the single biggest driver of your APR. A 760+ score often qualifies for the lender’s best advertised rate. A 660–719 score adds roughly 1–3 percentage points. A 580–659 score can add 5–10 points or more. Over a 60-month loan on a $30,000 car, the difference between a 6% APR and a 12% APR is more than $5,000 in interest.
Always get pre-approved through your bank or credit union before walking into the dealership. This gives you a baseline rate to negotiate against. Dealers earn money by marking up the lender’s rate (called the "dealer reserve"), so unless they can beat your pre-approval, take the bank financing. You can also use the pre-approval as leverage to push the dealer to match or beat it.
The 20/4/10 rule is a quick affordability check: put at least 20% down, finance for no more than 4 years, and keep total monthly transportation costs (loan payment + insurance + fuel + maintenance) under 10% of your gross monthly income. If a car fails this test, it’s a stretch — and a stretch on a depreciating asset compounds badly over a decade.
Mathematically, if you can confidently earn a higher return investing the cash than the loan’s APR, financing wins. But car loan rates today (7–8%) are competitive with long-term stock returns (~7–10%), and the loan is guaranteed cost while investment returns are not. For most high earners, paying cash for a reasonable car is the calmer, more compounding-friendly choice — it removes a fixed monthly drag on your income.
Most people optimize the wrong number. They negotiate $500 off the sticker price, then sign a 72-month loan at the dealer’s rate — and never do the math on what that decision costs over five years.
The HomeCFO Program teaches the framework that catches these decisions early — before you sign, not after. Same income. Different choices. Different decade.
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