Auto Loan Amortization Calculator and Payment Formula

An auto loan amortization calculator estimates your fixed monthly payment, total interest, and total payments from the amount borrowed, annual percentage rate (APR), and loan term. Enter the figures from a lender’s loan offer to see how each scheduled payment is divided between interest and principal over time.

Formula

The calculator uses the standard fixed-payment amortization formula. It converts the annual APR to a monthly rate, calculates the payment needed to repay the balance over the term, rounds that payment to cents, and then totals the scheduled payments.

• P is the amount financed, in dollars.
• APR is the annual percentage rate, entered as a percentage.
• r is the monthly rate: APR ÷ 1,200.
• n is the number of monthly payments: years × 12.
• M is the monthly payment, in dollars.

Payment formula: M = P × r ÷ [1 − (1 + r)⁻ⁿ]. The calculator displays M rounded to the nearest cent. Total interest: (M × n) − P, using the displayed, rounded payment. Total scheduled payments are M × n; they consist of the amount financed plus the calculated interest.

Worked Example

Using the defaults—$20,000 financed, 8% APR, and a five-year term—the monthly rate is 8 ÷ 1,200 = 0.0066667, and the number of payments is 5 × 12 = 60.

1. Substitute the values: M = 20,000 × 0.0066667 ÷ [1 − (1 + 0.0066667)⁻⁶⁰].
2. Evaluate the payment formula: the unrounded result is about $405.53, so the monthly payment rounded to cents is $405.53.
3. Multiply the displayed payment by the number of payments: $405.53 × 60 = $24,331.80 in scheduled payments.
4. Subtract the amount financed: $24,331.80 − $20,000 = $4,331.80 total interest.

Assumptions

• The interest rate is fixed for the full loan term.
• Payments are equal, made monthly, and applied at regular intervals.
• Interest accrues monthly at APR ÷ 12; the formula treats APR as a nominal annual rate.
• The calculation excludes fees, taxes, insurance, and other charges unless included in the amount financed. It does not model a down payment, trade-in, balloon payment, or optional products separately.

Reading an Auto Loan Amortization

At the start of a typical fully amortizing loan, more of each level payment goes toward interest because the outstanding balance is largest. As principal is repaid, the interest charged for each later period generally falls, and more of the same payment reduces principal. The scheduled payment can stay level even while its two components change.

An amortization schedule lays out this process one payment at a time. For each month, it shows the opening balance, interest accrued, payment, principal reduction, and remaining balance. A simplified monthly calculation is interest = opening balance × monthly rate; principal repaid = payment − interest; closing balance = opening balance − principal repaid. Actual lender calculations can use different conventions, so a generated schedule is an estimate.

The calculator’s total interest is the interest implied by the rounded monthly payment across all scheduled months. The final payment in a lender’s actual schedule may differ by a small amount because of rounding, payment dates, or the lender’s accrual method.

When to Use It and What to Enter

Use the calculator to understand the payment structure of a proposed fixed-rate auto loan or to compare arithmetic scenarios by changing one input at a time. Use the amount actually financed—not necessarily the vehicle’s advertised price. Depending on the transaction, the amount financed may reflect a down payment, trade-in value, taxes, fees, or products added to the loan. Check the lender’s written figures rather than guessing which items are included.

• Loan amount: Enter the principal borrowed in dollars. If the lender quotes an amount that includes financed charges, use that full amount.
• APR: Enter the annual percentage rate as a percentage, such as 8 for 8%, not 0.08. Get the rate and any applicable loan terms from the lender.
• Term: Enter the length in years. For a term stated in months, divide the months by 12; the calculator converts years to monthly payments.

To understand the effect of extending a term, compare the resulting payment and total interest while keeping the amount and APR unchanged. A longer term spreads repayment across more installments, which often lowers the calculated monthly payment but increases the number of months over which interest accrues. This is a mathematical comparison, not a recommendation about a particular loan.

What the Estimate Leaves Out

This is a basic fixed-rate model, not a complete vehicle purchase budget or a lender payoff quote. It does not account for a changing rate, irregular payment dates, late or missed payments, extra principal payments, payment holidays, or a final payment adjusted for rounding. It also does not determine a vehicle’s total ownership costs or whether a lender will approve an application.

Fees or other charges can affect the actual amount financed or the total paid. Taxes and insurance may be billed separately or handled in another way, and this calculator does not determine those arrangements. If a lender provides an itemized disclosure, use its payment schedule and stated totals for the exact transaction.

Common Calculation Mistakes

• Entering APR as a decimal: The input is a percent, so enter 8 for 8%, not 0.08. The formula divides the entered percentage by 1,200 to get a monthly rate.
• Using the wrong principal: A vehicle’s price, amount due at signing, and amount financed can be different figures. Check which one the loan documents specify.
• Mixing months and years: This calculator takes the term in years and multiplies it by 12. Do not enter a number of months as though it were years.
• Assuming the result is a quote: The formula does not include every lender-specific charge or timing convention. Small differences can result from rounding and payment-date details.

Frequently Asked Questions

What does an auto loan amortization calculator show?
It estimates a fixed monthly payment and, from that payment and the term, calculates scheduled total interest. A full amortization schedule also shows how each payment is divided between interest and principal.

Does a longer car loan term reduce total interest?
Not necessarily. For the same principal and APR, extending the term generally reduces the calculated monthly payment but adds more payment periods, which can raise total interest.

Why is my lender’s payment different from the calculator?
The amount financed, APR, term, fees, payment timing, and rounding may differ from the inputs or assumptions here. Compare the calculator inputs with the lender’s disclosure and payment schedule.

Does the monthly payment include insurance and taxes?
No. This calculation includes only principal and modeled interest. It does not add vehicle insurance, taxes, fees, or other charges.

Can I use this for extra payments?
This calculator assumes equal scheduled payments and does not model extra payments. Additional principal payments can change the payoff timeline and interest, depending on how the lender applies them.

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

Related

• Car Loan Calculator With Down Payment: Monthly Car Payment
• Auto Loan Calculator With Trade-In and Amount Owed (Negative Equity)

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