Monthly Car Loan Amortization Calculator and Formula

A monthly car loan amortization calculator estimates your fixed monthly principal-and-interest payment, total interest, and total payments from the amount borrowed, annual percentage rate (APR), and loan term. Enter those three figures to see a standard fixed-rate estimate; the calculation does not include fees or other vehicle costs.

Formula

The calculator applies the standard level-payment loan formula. It assumes the same payment is made once each month and the interest rate stays fixed for the full term:

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

• P is the amount financed, in dollars.
• r is the monthly interest rate as a decimal, calculated as APR ÷ 1,200. For an APR of 8%, r = 8 ÷ 1,200.
• n is the number of monthly payments, calculated as loan term in years × 12.

Total interest is the rounded monthly payment multiplied by the number of payments, minus the principal: total interest = monthly payment × n − P. The calculator rounds the monthly payment to cents before calculating total interest. The APR field is an annual percentage, not a monthly percentage.

Worked Example

Using the calculator’s default values—$20,000 financed, 8% APR, and five years—the monthly rate is 8 ÷ 1,200 = 0.0066667, and the term is 5 × 12 = 60 monthly payments.

1. Substitute the values: payment = 20,000 × 0.0066667 ÷ [1 − (1 + 0.0066667)⁻⁶⁰].
2. The unrounded result is approximately $405.53; rounded to the nearest cent, the monthly payment is $405.53.
3. Total of payments = $405.53 × 60 = $24,331.80.
4. Total interest = $24,331.80 − $20,000 = $4,331.80.

These figures are estimates under the stated assumptions. The calculation uses the cent-rounded payment when it totals the scheduled payments, which is why the total interest follows directly from the displayed monthly amount.

Assumptions

• The interest rate is fixed for the full repayment term.
• Payments are level, made monthly, and treated as occurring at the end of each monthly period.
• Interest accrues monthly using the APR divided by 12; the APR is entered as a percentage, so the formula divides it by 1,200.
• The estimate excludes fees, taxes, insurance, and other charges. It does not model a down payment, trade-in, optional add-ons, or a final payment that differs from the regular payments.

How Monthly Amortization Works

Amortization is the process of paying down a loan through scheduled payments that cover interest and reduce principal. In the early part of a standard fixed-payment loan, the outstanding balance is larger, so more of each payment goes toward interest. As the balance falls, the interest portion generally shrinks and more of the same payment goes toward principal.

For a particular month, a simplified schedule calculates interest as the balance at the start of the month multiplied by the monthly rate. The principal repaid is the payment minus that month’s interest. Subtracting the principal repaid from the starting balance gives the next balance. For example, on a $20,000 balance at the example monthly rate of 0.0066667, first-month interest is about $133.33; from a $405.53 payment, about $272.20 reduces principal.

A real contract may use specific day-count conventions, payment dates, rounding rules, or a final-payment adjustment. The calculator’s standard monthly model is useful for understanding the structure, but it cannot reproduce every servicing detail without those contract terms.

What the Results Tell You

The monthly payment is the estimated scheduled amount for principal and interest, not necessarily the whole amount due for owning or operating a vehicle. Total interest estimates the financing cost over the full scheduled term if payments are made as assumed. Total paid, in this example $24,331.80, includes principal and interest but not excluded charges.

The loan amount should represent the amount actually financed, not automatically the vehicle’s advertised price. A down payment or trade-in may reduce the amount financed, while financed taxes, fees, or add-ons may increase it. Use the figure in the lender’s written quote when available. Obtain the actual APR, term, fees, and payment details from the lender rather than treating the example rate as a current market figure.

Common Questions and Calculation Limits

A frequent misunderstanding is that APR divided by 12 captures every cost associated with borrowing. It supplies the periodic rate for this simplified payment calculation, but this calculator does not separately itemize fees or determine how a lender treats them. Another common mix-up is entering 8 as a monthly rate: enter 8 in the APR field for 8% per year, not 0.08 or a monthly percentage.

Results also depend on the term and the amount financed. Holding the other inputs constant, a longer term means more scheduled payments and can increase total interest, even though the monthly payment formula spreads repayment over more months. This is a mathematical consequence of the inputs, not a recommendation about which term to use.

The formula assumes a positive APR from 0.01% to 100% and a term from 0.0833 to 50 years, as allowed by the calculator fields. It does not calculate a zero-interest loan, variable-rate loan, irregular payments, skipped payments, extra principal payments, refinancing, or early payoff. To examine those situations, the schedule would need additional inputs and assumptions.

Frequently Asked Questions

How do I calculate a monthly car loan payment?
Convert the APR to a monthly decimal by dividing it by 1,200, convert the term to months, and use the level-payment formula shown above. This calculator performs those steps and rounds the payment to cents.

Does the monthly payment include insurance and taxes?
No. The estimate covers principal and interest only. Insurance, taxes, fees, and other vehicle-related costs are not included unless they are part of the amount financed, and the calculator does not identify or itemize them.

Why is the first payment mostly interest?
Interest for a period is calculated on the outstanding balance. At the beginning, the balance is relatively high; as principal is repaid, the balance and the interest portion generally decline.

Can I use this calculator for a zero-interest loan?
No. The displayed payment formula divides by the monthly rate, and the APR input has a minimum of 0.01%. For a genuinely zero-interest loan with equal payments, principal divided by the number of payments gives the basic payment, subject to the contract’s rounding and payment terms.

Why might a lender’s payment differ from this estimate?
The amount financed, APR, term, payment timing, fees, rounding, and contract-specific calculations may differ from the inputs or assumptions here. Compare the calculator’s assumptions with the lender’s written payment disclosure.

The 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