Car Loan Interest Rate Calculator and Payment Formula

This car loan calculator estimates your monthly payment and total interest from the amount borrowed, annual percentage rate (APR), and loan term. Enter the figures from a lender’s offer to see how they translate into scheduled payments; the result is an estimate, not a loan quote.

Formula

The calculator uses the standard fixed-rate, fully amortizing loan payment formula:

Monthly payment = P × r ÷ [1 − (1 + r)⁻ⁿ]

• P is the amount financed, in dollars.
• r is the monthly interest rate as a decimal: APR ÷ 12 ÷ 100.
• n is the number of monthly payments: loan term in years × 12.
• APR is the annual percentage rate entered as a percentage.

Total interest is calculated as the rounded monthly payment multiplied by the number of payments, minus the amount borrowed: total interest = monthly payment × n − P. The payment is rounded to cents before total interest is calculated. This formula assumes a regular payment schedule and does not add separate charges to the loan.

Worked Example

Using the calculator’s default values—$20,000 financed, 8% APR, and five years—the calculation is:

1. Convert APR to a monthly decimal rate: 8 ÷ 1,200 = 0.0066667.
2. Find the number of payments: 5 × 12 = 60.
3. Apply the payment formula: $20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰] = $405.53 per month, rounded to the nearest cent.
4. Calculate total interest using the rounded payment: $405.53 × 60 − $20,000 = $4,331.80.

These are scheduled-payment estimates under the stated assumptions. A lender’s exact payment can differ if its amount financed, APR, payment dates, or included charges differ.

Assumptions

• The interest rate is fixed for the full term.
• Payments are equal monthly payments, made at the end of each monthly period.
• Interest accrues monthly using APR divided by 12.
• Fees, taxes, insurance, optional products, and other charges are not included unless already reflected in the loan amount or rate entered.

What Determines a Car Loan Rate?

There is no single interest rate that applies to every car loan. The APR is a lender-provided input, and a reader should use the rate shown for the particular offer being evaluated rather than treating a calculator default as a market quote. The actual offer depends on the lender’s assessment and the loan’s details; the calculator cannot predict approval or determine what rate a person will receive.

When comparing written offers, check that the figures refer to the same kind of borrowing and the same assumptions. In particular, distinguish the APR from a stated interest rate if the offer presents both, and confirm the amount financed, repayment term, and any charges included. Do not infer that a lower monthly payment alone means a lower borrowing cost: a longer term can reduce each scheduled payment while increasing the total interest paid.

How to Use the Estimate

Enter the amount actually financed, not automatically the vehicle’s advertised price. A down payment or trade-in can reduce the amount borrowed, while taxes, fees, or optional products may increase it if they are financed. Use the loan term in years and APR as a percentage—for example, enter 8 for 8%, not 0.08.

Use the results to understand the arithmetic and compare scenarios on equal terms. For example, hold the loan amount and APR constant while changing the term to see the effect on payment and interest; then restore the term and vary the APR. This illustrates how inputs affect the formula, but it does not establish which arrangement is suitable for a particular person.

The total-interest output is the sum of the modeled payments above the original principal. It is not the total amount paid: add the principal to the interest to get the modeled total of payments. With the default values, that total is $24,331.80 ($20,000 principal plus $4,331.80 interest), subject to the cent-rounded payment assumption.

Common Sources of Difference

• Using the wrong principal: the financed amount can differ from the car’s price because a down payment, trade-in, or financed charges change the balance.
• Confusing APR and a monthly rate: enter annual APR as a percentage. The formula converts it to a monthly rate by dividing by 1,200.
• Rounding: payments are rounded to cents here. A lender’s payment schedule may handle the final payment or daily interest differently.
• Changing payment timing: this estimate assumes regular monthly periods. Irregular first-payment dates or other schedules can alter interest calculations.
• Leaving out charges: insurance, taxes, and fees are not automatically included. They affect the estimate only if incorporated into the entered amount or otherwise reflected in the terms.

A related misconception is that a car loan’s interest is always a flat percentage of the original balance each month. In a standard amortizing loan, interest is calculated on the outstanding balance; as payments reduce that balance, the interest portion generally declines and more of a level payment goes toward principal. The calculator summarizes the scheduled payment and total interest, but it does not display a month-by-month amortization table.

Frequently Asked Questions

What is the interest rate on a car loan?
It is the rate charged for borrowing to finance a vehicle. The applicable rate varies by offer and borrower, so use the APR supplied in the specific lender disclosure; the calculator’s 8% default is only an example input.

Is APR the same as the interest rate?
Not necessarily. APR is an annualized measure used in a loan offer and can account for certain costs under the offer’s terms, while a stated interest rate describes the rate used to accrue interest. Check the disclosure to see which figure applies to the payment calculation.

How do I calculate a car loan payment?
Convert annual APR to a monthly decimal rate by dividing by 1,200, multiply the amount financed into the amortizing-payment formula, and use the number of monthly payments as the exponent. This calculator performs those steps and rounds the monthly result to cents.

Does a longer car loan term lower interest?
A longer term often lowers the scheduled monthly payment for the same principal and APR because the balance is repaid over more payments. It can increase total interest, since interest accrues for more months. The calculator shows both measures for the inputs entered.

Why is my lender’s payment different from this estimate?
The lender may use a different amount financed, APR, payment date, rounding method, or included charges. Compare the entered figures with the written offer and ask the lender to explain any difference in its disclosed payment schedule.

This result is an estimate, and the lender’s disclosure gives the exact figures.

Related

• Interest Rate Hike Calculator: Estimate Payment Changes
• Savings Interest Rate Hikes: How They Affect Your Money
• Interest Rate Hedging Calculator and How It Works

in p | Loan amount | $ | 20000 | 1 | 100000000
in apr | Interest rate (APR) | % | 8 | 0.01 | 100
in yrs | Loan term | years | 5 | 0.0833 | 50
out pmt | Monthly payment | $ | p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2