Car loans: how to calculate the payment and interest

To calculate a car payment, start with the amount financed—not the vehicle’s sticker price. The down payment reduces the principal, while financed fees, insurance or add-ons can increase it.

Calculate the financed amount

amount financed = vehicle price - down payment

For a $320,000 vehicle and a $64,000 down payment:

320000 - 64000 = 256000

Use $256,000 as the base principal unless other costs are rolled into the loan.

Fixed-payment formula

payment = P x [r(1 + r)^n] / [(1 + r)^n - 1]

P is the amount financed, r the monthly rate and n the number of payments.

Worked example

Assume:

• Vehicle price: $320,000
• Down payment: $64,000
• Amount financed: $256,000
• Annual rate: 12%
• Term: 48 months
• Origination fee: 2%

The estimated base payment is $6,741.46; interest over the term is about $67,590.17. The fee equals $5,120, producing an estimated cost of $328,710.17 for the financing, before insurance.

These amounts are in the same currency as the vehicle price. They do not include the $64,000 down payment in the financing-cost total.

Insurance, fees and add-ons

Insurance may be paid upfront, monthly, financed or purchased separately. Accessories, registration expenses, warranties and fees can also be added to principal and then generate interest. Ask for both the net amount financed and the total amount payable.

When comparing dealer and bank offers, review the same down payment, principal, rate, term, fees, insurance, total cost and prepayment rules. In Mexico, compare the official CAT as well.

Use the loan calculator with the auto-financing option. The guides to monthly loan insurance and comparing two loans help uncover costs beyond the headline payment.

Frequently asked questions

Does a larger down payment reduce interest?

Generally, yes. It lowers the principal on which interest is calculated.

Which car-loan term is best?

There is no universal answer. A longer term lowers the payment but normally increases total interest and may keep you in debt while the vehicle depreciates.

Is insurance included in the payment?

Only if the contract says so. Confirm whether it is separate, monthly or financed.