How a car loan payment is calculated
A car loan is a standard amortizing loan: you borrow the vehicle price minus whatever you pay up front, and repay it in equal monthly installments over a fixed term. Each payment covers that month's interest on the outstanding balance first, with the remainder reducing the balance. The calculator takes the vehicle price, your down payment, the annual interest rate and the term in months, and returns the monthly payment, the total you will pay, and how much of that is interest.
Principal = Price − Down payment
M = Principal × r(1 + r)ⁿ / ((1 + r)ⁿ − 1)r is the monthly rate (annual APR ÷ 12) and n is the number of monthly payments. A trade-in works exactly like a down payment: subtract its value from the price before running the numbers.
Worked example
A car priced at $32,000 with a $5,000 down payment, financed at 6.5% APR over 60 months. The principal is $27,000 and the monthly rate is 0.065 / 12 ≈ 0.005417.
M = 27000 × 0.005417 × (1.005417)⁶⁰ / ((1.005417)⁶⁰ − 1) ≈ $528.30Over five years you pay about $31,700, so the loan costs roughly $4,700 in interest. Stretching the same loan to 72 months drops the payment to about $454 but raises total interest to around $5,700 — a common trade-off dealers present as “lower monthly” without mentioning the extra thousand dollars.
What term length really costs
| Term | Monthly payment | Total interest | Notes |
|---|---|---|---|
| 36 months | $827 | $2,780 | Cheapest overall; highest payment |
| 48 months | $640 | $3,730 | Common sweet spot for new cars |
| 60 months | $528 | $4,700 | Most popular US term |
| 72 months | $454 | $5,690 | Often underwater for the first 2–3 years |
| 84 months | $401 | $6,700 | Loan may outlast the car's reliable life |
Figures use the $27,000 / 6.5% example. Longer terms also increase the time you owe more than the car is worth (“negative equity”), because cars depreciate fastest in the first years while a long loan pays down principal slowly.
New vs used car rates
Lenders charge more for used vehicles because their value is harder to predict and they are worth less as collateral. As a rough guide in the US market, a borrower with good credit might see 5–8% on a new car and 7–12% on a used one; subprime borrowers can pay 15–20%+. Manufacturer-subsidised financing (0–2.9% offers) is usually only available on new vehicles and often replaces a cash rebate, so compare both options with the calculator: a rebate that lowers the price can beat a low rate on a bigger principal.
How to lower the total cost
- Put more down. Every $1,000 of down payment on the example saves about $175 in interest and cuts the payment by roughly $20/month. A 20% down payment on a new car also keeps you out of negative equity.
- Get pre-approved before visiting the dealer. A bank or credit union quote gives you a rate to beat; dealer financing frequently adds 1–2 points as a markup.
- Choose the shortest term you can afford. If the 48-month payment fits, do not take 72 just to free up cash — the difference is pure interest.
- Round up your payment. Paying $550 instead of $528 on the example clears the loan about three months early with no penalty on most US auto loans.
- Skip add-ons rolled into the loan. Extended warranties, GAP insurance and paint protection financed over five years accrue interest too. Buy them separately if you want them.
Fees and taxes the calculator does not include
Sales tax, registration, documentation fees and dealer “prep” charges are commonly added to the amount financed. In a state with 7% sales tax, a $32,000 car adds $2,240 to the loan before you have driven it. Enter the out-the-door price rather than the sticker price to see a realistic payment. If your state taxes the trade-in difference rather than the full price, the tax is lower — check your local rules.