How Is Auto Loan Interest Calculated?

This calculator estimates the monthly payment and total interest on a fixed-rate auto loan from the amount financed, annual percentage rate (APR), and term. Enter those figures to see the scheduled payment and the interest paid if every payment is made on time and the loan runs to term.

Formula

For a fixed-rate loan with equal monthly payments, the payment is calculated by amortization:

Payment = P × r ÷ [1 − (1 + r)⁻ⁿ]

• P is the amount financed, in dollars. This is the principal being borrowed, not necessarily the vehicle’s advertised price.
• r is the monthly interest rate as a decimal: annual APR ÷ 1,200. For an 8% APR, r = 8 ÷ 1,200, or about 0.006667 per month.
• n is the number of monthly payments: loan term in years × 12.

The calculator rounds the monthly payment to the nearest cent. Total interest is then estimated as the rounded payment multiplied by the number of payments, minus the principal: Total interest = payment × n − P. This assumes the same payment is made for every scheduled month and does not model a final adjustment, fees, or changes to the balance.

Worked Example

Using the calculator’s default inputs—$20,000 financed, 8% APR, and a five-year term:

1. Convert APR to a monthly decimal rate: 8 ÷ 1,200 = 0.0066667.
2. Convert the term to payments: 5 × 12 = 60 months.
3. Apply the payment formula: $20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰] = approximately $405.53 per month after rounding.
4. Calculate scheduled payments: $405.53 × 60 = $24,331.80.
5. Subtract the amount financed: $24,331.80 − $20,000 = $4,331.80 total interest.

The displayed monthly payment is the formula’s headline result; the interest figure is the estimated total over the full term using that rounded payment.

Assumptions

• The APR and payment amount stay fixed throughout the term.
• Payments are equal, made monthly, and applied at regular intervals.
• Interest is modelled monthly at APR ÷ 12; actual lender calculations and payment posting can differ.
• The estimate excludes fees, taxes, insurance, optional products, and other costs unless they are included in the amount financed.

How Auto Loan Interest Accrues

Auto loans are commonly amortizing loans: each scheduled payment covers interest due for the period and reduces principal. Interest is calculated on the outstanding balance, so the interest portion generally declines as principal is paid down. Early in the schedule, the balance is larger, and a greater share of a level payment goes toward interest. Later, more of each payment reduces principal.

A simplified monthly illustration shows the mechanism. At the start of a period, multiply the unpaid principal by the monthly rate to estimate that period’s interest. Subtract that interest from the payment to find the principal reduction. Then subtract the reduction from the prior balance. With a $20,000 balance and a monthly rate of 0.0066667, one month’s interest is about $133.33; from a $405.53 payment, about $272.20 would reduce principal. Lender methods, payment dates, and rounding can produce different exact figures.

What Changes the Interest Cost?

• Amount financed: A larger principal generally produces a larger payment and more interest when APR and term are unchanged. A down payment or trade-in may reduce the amount borrowed, while financed taxes or add-ons may increase it.
• APR: The APR is the annual rate used here as an input, converted to a monthly rate for the calculation. Use the figure supplied for the specific loan; do not substitute a rate from a general estimate.
• Term: A longer term spreads repayment over more months. It can lower the scheduled payment in many fixed-rate examples, but leaves the balance outstanding longer and can increase total interest.
• Payment timing and extra payments: Paying principal sooner can reduce later interest when the contract applies the extra amount to principal. The effect depends on lender handling and any contract terms; this calculator assumes only regular scheduled payments.

APR, Interest Rate, and Amount Financed

The interest rate describes the rate used to calculate interest on the balance. APR is an annualized cost measure that may reflect certain borrowing costs as well as the interest rate, depending on how the loan is presented. For an estimate, enter the lender’s stated APR, but check the loan disclosure for the rate, financed fees, payment schedule, and total of payments. This calculator treats APR as a nominal annual rate divided by 12, which is a useful standard estimate rather than a reproduction of every lender’s method.

The vehicle’s price is not automatically the loan principal. The amount financed can differ because of a down payment, trade-in credit, taxes, fees, or optional products included in the transaction. To estimate the loan’s interest accurately, use the amount actually being financed—not just the sticker price.

Limits and Common Calculation Mistakes

• Confusing monthly and annual rates: The formula needs a monthly decimal, so APR is divided by 1,200. Using 8 instead of 0.08 per year, or dividing by 100 but not 12, gives a wrong result.
• Using the wrong principal: Enter the financed balance, not the vehicle price before down payment or other adjustments.
• Ignoring rounding: A real payment is stated in cents. Multiplying an unrounded formula result by all months can differ slightly from totals based on the rounded installment. The calculator uses the rounded payment for its interest estimate.
• Assuming every loan follows this schedule: Variable rates, irregular payments, deferred payments, late payments, daily-interest methods, and payoff quotes are not represented. A payoff amount on a particular date can differ from the remaining scheduled payments because interest accrues over time and contract details matter.

Use the result to understand the mathematics and compare the figures with a written loan disclosure. The calculation does not determine approval, account for a particular lender’s terms, or include costs not entered as principal.

Frequently Asked Questions

Is auto loan interest calculated monthly or daily?
This estimate converts APR to a monthly rate and models monthly accrual. Some lenders calculate interest using a daily balance or another contract-specific method, so the actual allocation can vary.

Does paying extra reduce total auto loan interest?
Extra money applied to principal can reduce the balance on which later interest is calculated. Confirm how the lender applies extra payments; this calculator does not include extra payments or predict a payoff date.

Why is more of my early payment interest?
Interest for each period is based on the outstanding balance. Since the balance is highest near the beginning, interest takes a larger share of a level payment then; as principal falls, that share generally shrinks.

Is the total interest the same as the total cost of the car?
No. Total interest is only the estimated borrowing cost in this model. The vehicle price and other transaction costs are separate, and the calculator excludes fees and expenses not included in the amount financed.

Can I use this to calculate an early payoff amount?
No. It estimates interest across the full scheduled term. A payoff quote depends on the unpaid balance, date, interest accrued, and the loan’s terms; request the exact figure from the lender.

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

Related

• Advanced Loan Payment Calculator: Payments and Interest
• Calculate a Monthly Loan Payment: Formula and Example
• How to Calculate Loan Payments: Formula and Example

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 | $ | round(p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2