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Car loan interest is calculated on the balance still unpaid

Most auto loans recalculate interest as the balance falls, shifting each scheduled payment from interest toward principal.

Sal Moretti

By Sal Moretti · Money Reporter

4 min read

Car loan interest is usually calculated on the unpaid principal, the amount you still owe. On a typical amortized loan, the lender calculates that period’s interest from the current balance, takes it from your scheduled payment, and applies the rest to principal.

That is why the same monthly payment can be interest-heavy at the start and principal-heavy near the finish. The key figures are the amount financed, annual rate, loan term and payment schedule.

How car loan interest is calculated each month

Most lenders offer simple-interest auto loans. Under that setup, the charge is based on the principal still owed each month, rather than a flat charge based on the original balance for the entire term.

Amortization is the payment process that steadily reduces that balance.

  1. Start with the unpaid principal balance.
  2. Convert the annual interest rate to a monthly rate by dividing the decimal rate by 12.
  3. Multiply that monthly rate by the current balance to estimate that month’s interest.
  4. Subtract the interest charge from the scheduled payment. The remainder pays down principal.
  5. Use the lower principal balance to calculate the next month’s interest.

A first-month estimate

Example: A $20,000 balance with a 5% annual rate.

  • Annual rate as a decimal: 0.05
  • Monthly rate: 0.05 ÷ 12 = 0.004167
  • First month’s interest: 0.004167 × $20,000 = $83.34

Subtracting $83.34 from the scheduled payment would show that month’s principal payment. The next calculation starts with a smaller balance, so the interest share generally falls.

Do not estimate total interest on a standard amortized loan by multiplying the original balance by the rate and number of years. That shortcut misses the shrinking balance. A lender’s amortization schedule or a loan calculator is better suited to estimating the payment split and total interest.

The terms behind the payment

  • Principal: the amount borrowed and still unpaid.
  • Interest rate: the percentage charged for borrowing the money.
  • Loan term: the time allowed to repay the loan, usually stated in months.
  • Monthly payment: the scheduled amount due each month, generally divided between interest and principal.
  • APR: annual percentage rate. It can include the interest rate and certain lender fees, giving a broader view of borrowing cost.

The stated interest rate is the figure used for a period-by-period interest calculation. APR can help compare loans with fees, but do not assume it is interchangeable with the rate in every calculation. Check the loan disclosure, then compare the stated rate, APR, amount financed, fees and total of payments together.

Same payment, changing split

A scheduled payment can stay the same each month while its allocation changes. Early in the loan, the balance is higher and the interest charge is larger. As payments reduce the balance, more of the payment goes to principal.

One credit-union illustration puts a $40,000 loan over 60 months at 6% APR at a stated $773 monthly payment, $6,399 in total interest and $46,399 paid overall. Those figures are an example, not a quote for every borrower or loan.

A longer term can lower the monthly payment by spreading repayment over more months. It generally raises total interest because the balance remains outstanding longer.

One contract detail that can affect early payoff

Ask whether the loan uses simple interest or precomputed interest. Precomputed-interest loans are an alternative in which interest costs are front-loaded, and early payoff may save less than it would on a simple-interest loan.

Read the payoff and prepayment terms in the contract before relying on an early-payoff estimate.

A quick checklist for comparing car-loan offers

  • Amount financed, including taxes or fees rolled into the loan
  • Stated interest rate and APR
  • Loan term in months
  • Monthly payment
  • Total of payments and total interest shown in the disclosure or amortization schedule
  • Whether interest is simple or precomputed, plus prepayment terms

The rate offered can reflect credit scores and history, income and debts, the loan amount and term, down payment relative to vehicle value, and whether the car is new or used, according to the Consumer Financial Protection Bureau. The CFPB recommends comparing lenders and says that keeping rate shopping within 14 to 45 days generally lets multiple credit checks count as one inquiry.

Frequently asked questions

Why does more of my car payment go to interest at first?

On a typical simple-interest, amortized car loan, interest is calculated from the remaining balance. The balance is largest at the beginning, so the interest charge is larger then. As the balance falls, more of the scheduled payment goes to principal.

Does paying off a car loan early reduce the interest I pay?

It can on a simple-interest loan because future interest is calculated from the unpaid balance. A precomputed-interest loan may save less with early payoff, so check the payoff and prepayment terms in the contract.

What should I compare besides the monthly car payment?

Compare the amount financed, stated interest rate, APR, term, total of payments or total interest, and listed fees and prepayment terms. A longer term can lower the monthly payment while generally increasing total interest.

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