How Car Loan Interest Works: Rates, APR and Loan Term

The short answer

Most car loans charge simple interest on the balance you still owe. Each month, interest is worked out on that balance, your payment covers the interest first, and the rest pays down the loan. The Consumer Financial Protection Bureau says simple interest is far more common than the alternative, and that it is calculated on the outstanding balance on a daily or monthly basis.

Three things decide how much interest you pay in total: how much you borrow, the rate, and how long you take to pay it back. The term is the one people underestimate. On a $30,000 loan at 7.5%, stretching from 48 to 72 months lowers the payment by about $207 a month and adds about $2,529 of interest.

How one month of interest is calculated

For a loan with monthly interest, the monthly rate is the annual rate divided by 12:

This month’s interest = balance × (annual rate ÷ 12)

Take $30,000 at 7.5% for 60 months. The standard fixed-payment formula gives a payment of $601.14.

• Month 1 interest: $30,000 × 0.075 ÷ 12 = $187.50
• Principal paid: $601.14 − $187.50 = $413.64
• New balance: $30,000 − $413.64 = $29,586.36

Month 2 starts from the lower balance, so the interest is a little smaller and a little more of the same $601.14 goes to principal. That pattern continues until the final payment, when interest is only a few dollars. The CFPB’s explainer on amortization describes the same thing: at the start of the loan, more of each payment generally goes to interest than to principal.

For reference, the payment formula is:

Payment = P × r ÷ (1 − (1 + r)⁻ⁿ)

where P is the amount borrowed, r is the annual rate ÷ 12, and n is the number of months.

If your loan accrues interest daily, the idea is the same with a smaller step. At 7.5% on $30,000, a day of interest is about $30,000 × 0.075 ÷ 365 = $6.16. That is why paying a few days early or late can nudge how much of a payment goes to principal.

The term: lower payment, higher total

Here is the same $30,000 at 7.5% over common terms:

Term	Monthly payment	Total interest	Total paid
36 months	$933.19	$3,594.69	$33,594.69
48 months	$725.37	$4,817.58	$34,817.58
60 months	$601.14	$6,068.30	$36,068.30
72 months	$518.70	$7,346.68	$37,346.68
84 months	$460.15	$8,652.41	$38,652.41

Going from 36 to 84 months cuts the payment in half (from $933 to $460) but more than doubles the interest (from $3,595 to $8,652). Each extra year adds roughly $1,250 to $1,300 of interest, and that is at the same 7.5% rate for every term. If a longer term comes with a higher rate on your offer, the gap is wider still.

The CFPB makes the same point in its guide to comparing auto loan offers: focus on the total cost over the full term, not just the monthly payment. It notes that some financial experts recommend an auto loan of five years or less, and that longer loans are more likely to leave you owing more than the car is worth.

Interest rate vs APR

The interest rate is what the lender charges you to borrow. The CFPB describes the APR as the interest rate plus additional fees charged with the loan, such as origination charges. Under the Truth in Lending Act, lenders must disclose the APR before you finalize a car loan, which is why the CFPB says to compare APRs with APRs, not APRs with interest rates.

A worked example shows why. Say one lender offers 7.0% and adds a $595 loan fee to the balance, so you borrow $30,595 for 60 months:

• Payment at 7.0% on $30,595: $605.82 a month.
• The same $605.82 a month on the $30,000 you actually use works out to an annual rate of about 7.83%.

So a “7.0%” loan with a fee costs about the same each month as a 7.83% loan with no fee. Another lender quoting 7.5% with no fee ($601.14 a month) would be the cheaper deal, even though its headline rate is higher. That is the comparison the APR is designed to make for you.

Simple interest vs precomputed interest

The CFPB describes a second, less common way of charging interest: precomputed interest, where the total interest is worked out up front, added to the amount borrowed, and divided into monthly payments. Under that method, the CFPB says, making extra payments does not reduce the principal or the interest owed.

With simple interest, extra payments do help. Paying more than the monthly amount reduces the principal, and future interest is charged on the smaller balance. The CFPB’s advice: if you plan to pay a loan off early, make sure the lender is using simple interest.

What an extra $50 a month does

On the $30,000 at 7.5% over 72 months, adding $50 to every payment and applying it to principal:

• Payoff in 65 months instead of 72.
• Total interest $6,513.35 instead of $7,346.68, a saving of $833.33.

Before you do this, check two things. The CFPB says your contract and state law determine whether you can pay early without a penalty. And ask your lender how an extra payment is applied, so you know it is reducing the principal rather than just paying a future installment early.

What else changes the interest you pay

• The amount financed. Sales tax and fees rolled into the loan are borrowed money too, and you pay interest on them for the full term. A bigger down payment or trade-in shrinks the balance from day one.
• Your rate. A difference of even half a point shows up in every month of the loan. The CFPB notes that asking questions before you shop makes you more likely to get the best rates and terms for your budget.
• Negative equity. Rolling an old loan balance into a new one increases what you borrow, and with it the interest and the chance of owing more than the car is worth.

Common mistakes

• Shopping by monthly payment. A lower payment can simply mean a longer term. Compare total cost.
• Comparing a rate with an APR. Make sure both offers quote APR.
• Assuming extra payments go to principal. Confirm how the lender applies them.
• Ignoring the fees in the amount financed. A $595 fee financed over 60 months costs more than $595.
• Choosing the longest term “for flexibility.” You can usually pay a simple-interest loan faster, but you are locked into the higher total if you do not.

When to get help

Lenders must give you Truth in Lending disclosures, including the APR, before you finalize the loan. Read them before you sign. If the numbers do not match what you were told, ask for an explanation. The CFPB suggests asking the lender or dealer whether the contract has a prepayment penalty, and notes that some states prohibit such penalties for certain loans. If you are rolling negative equity into a new loan or the contract terms are unclear, it is worth having someone you trust read it with you.

To see your own numbers, the car loan calculator adds sales tax, fees, down payment and trade-in, and prints a payment schedule. The loan calculator shows the full amortization table and what extra monthly payments save.

General information, not financial advice. Your loan contract and Truth in Lending disclosure are the final word on your rate, fees and payoff terms.