How your monthly car payment is calculated
Auto loans are amortizing loans, which means each monthly payment is split between paying interest on the outstanding balance and reducing the principal. Early payments contain more interest than later ones because the balance is highest at the start. As principal falls, the interest portion of each payment shrinks and principal repayment accelerates.
The amount financed is not simply the vehicle price. In many markets, a purchase tax such as sales tax, VAT, or GST is added to the taxable price, while a down payment and trade-in value are subtracted. For example, a vehicle priced at 25,000 with an 8% purchase tax becomes 27,000 before adjustments. Subtract a 3,000 down payment and 2,000 trade-in, and the financed balance is 22,000. That adjusted figure feeds into the amortization formula.
Tax treatment is not identical everywhere. Some jurisdictions reduce the taxable base when a trade-in is applied, some tax fees separately, and others fold taxes into the displayed vehicle price. That is why this calculator is best treated as a practical estimator calculator: the finance maths is consistent, but local tax rules still matter.
When the interest rate is zero, the monthly payment is simply the loan amount divided by the number of months. For any positive rate, the standard amortization formula is used.
Loan amount = (Vehicle price + Purchase tax) - Down payment - Trade-in value
The financed balance is the vehicle price plus applicable purchase tax, reduced by any upfront cash contribution and trade-in equity.
M = L x r / (1 - (1 + r)^(-n))
M is the monthly payment, L is the loan amount, r is the monthly interest rate (annual rate / 12), and n is the total number of monthly payments.
Total interest = (M x n) - L
Total interest is the sum of all payments minus the original loan amount. This is the true cost of borrowing.