A $30,000 auto loan at 6.5% APR over 60 months comes out to about $587 a month, with roughly $5,220 in total interest by the time it's paid off. That number comes from the same amortization formula used for any installment loan, but auto loans add a few extra pieces — trade-in value, sales tax, and fees — that change what you're actually financing. This guide walks through the math with real figures, shows how term length trades a lower payment for more interest, and points you to calculators that do the arithmetic for you.
The formula behind every auto loan payment
Car loans use the standard fixed-rate installment formula: M = P[r(1+r)^n] / [(1+r)^n − 1]. M is the monthly payment, P is the amount financed, r is the monthly interest rate (your annual APR divided by 12), and n is the number of monthly payments (loan term in years times 12).
It's the identical formula behind a mortgage — only the size and term of the loan differ. A car loan usually runs 3 to 7 years instead of 15 or 30, which is why the interest portion is smaller in dollar terms even though the rate is often similar or higher than a mortgage rate.
A worked example: $30,000 over 5 years
Finance $30,000 at 6.5% APR for 60 months. The monthly rate is 6.5% ÷ 12 = 0.5417%, and there are 60 payments. Running those through the formula gives a monthly payment of about $587. Over 60 months you pay roughly $35,220 total, meaning about $5,220 of that is interest.
Stretch the same $30,000 loan to 72 months at the same rate and the payment drops to about $504 a month — a saving of roughly $83 a month. But total interest climbs to about $6,309, which is around $1,090 more than the 5-year loan. The lower payment isn't free; you're renting the extra room in your budget by paying more for the car overall.
What actually goes into your loan amount
The amount you finance is rarely just the sticker price. Say you're buying a $32,000 car with a $4,000 down payment and a $3,000 trade-in, and your state charges 6% sales tax on the price after the trade-in credit. Tax works out to 6% × ($32,000 − $3,000) = 6% × $29,000 = $1,740.
Your amount financed is then $32,000 − $4,000 down − $3,000 trade-in + $1,740 tax = $26,740. Documentation fees, title and registration charges, and any add-ons like an extended warranty typically get rolled in the same way, so it's worth listing every line item before you assume you know your real loan amount.
Term length and down payment are the two levers you control
A bigger down payment or trade-in lowers the amount financed directly, which lowers both the monthly payment and the total interest — no trade-off involved. On the $30,000 example above, adding $3,000 more down (financing $27,000 instead) at the same rate and term brings the payment down to about $528 a month, a saving of roughly $59 a month with less interest paid overall.
Term length is a genuine trade-off, not a free lunch: shorter terms mean higher payments but far less interest, while longer terms mean lower payments but more interest and a longer stretch where you can owe more than the car is worth. A widely used rule of thumb is the 20/4/10 guideline — aim for at least 20% down, a loan term no longer than 4 years, and total monthly transportation costs (payment, insurance, and fuel combined) under 10% of your gross income.
Run the numbers yourself
Frequently Asked Questions
How much should I put down on a car?
20% is a common target, especially on a new car, because it helps offset the steep first-year depreciation and keeps you from owing more than the car is worth. Any amount you put down still helps — it reduces the loan principal dollar for dollar, which lowers both your payment and your total interest.
Does a longer loan term actually save me money?
It lowers your monthly payment but not your total cost. In the example above, moving from a 60-month to a 72-month term on a $30,000 loan cuts the payment by about $83 a month but adds roughly $1,090 in extra interest over the life of the loan. Choose the shortest term your budget can comfortably handle.
Is a car loan calculated the same way as a mortgage?
Yes — both use the same fixed-rate amortization formula with principal, rate, and term as the inputs. The difference is what makes up the principal: a car loan's amount financed typically includes sales tax, fees, and trade-in value, while a mortgage's principal is simply the home price minus the down payment.