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Auto loans use a fixed-rate amortizing payment formula. Each month, you pay a fixed amount; part of it goes to interest on the remaining balance, the rest goes to principal.
The formula:
P = L × (r × (1 + r)n) ÷ ((1 + r)n − 1)
Where P is the monthly payment, L is the loan amount, r is the monthly interest rate (APR ÷ 12), and n is the number of months. The output rounds to the cent and is what your lender will quote.
Included: principal, interest, term length, down payment, trade-in equity. These are the four levers that determine the payment.
Not included: sales tax, registration, dealer fees, GAP insurance, extended warranty, doc fees. These vary by state and dealer and are typically rolled into the financed amount or paid up front. Add them to the "vehicle price" field to see the impact on payment.
A common pitch from dealer finance managers: "We can get the payment to $X by going to 84 months." It works — the math is real — but here's what it costs:
| Term | Monthly @ 7% on $30k | Total interest paid |
|---|---|---|
| 36 months | $926 | $3,348 |
| 48 months | $719 | $4,499 |
| 60 months | $594 | $5,651 |
| 72 months | $512 | $6,829 |
| 84 months | $453 | $8,033 |
The 84-month payment is $141/month lower than the 60-month — but you pay an extra $2,400 in interest. Plus you're underwater on the loan for years longer, which makes selling or trading the car painful.
The traditional rule: total monthly transportation costs (loan + insurance + gas + maintenance) under 15% of take-home pay. For a borrower making $5,000/month after tax, that means $750/month total — leaving about $450–$550 for the loan payment itself.
Use the calculator backwards: pick a comfortable monthly payment, plug in a realistic APR for your credit, then solve for vehicle price. Stick to that number on the lot.