Car Loan Calculator Monthly Payment APR Formula

A car loan calculator estimates your monthly principal-and-interest payment from the amount financed, APR, and loan term; enter those three figures to see the payment and total interest. It uses a fixed-rate, level-payment formula, so the result is an estimate rather than a lender’s final quote.

Formula

The monthly payment formula is:

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, or APR ÷ 1,200 when APR is entered as a percentage.
• n is the number of monthly payments: loan term in years × 12.

The calculator’s total-interest estimate is monthly payment × number of payments − amount financed. It calculates the payment from the unrounded formula and displays it to the nearest cent; the total-interest output uses that displayed, cent-rounded payment. Small differences from a lender’s schedule can therefore occur if the lender handles rounding or payment dates differently.

Worked Example

Using the defaults—$20,000 financed, 8% APR, and a five-year term—convert the APR and term first:

• Monthly rate: 8 ÷ 1,200 = 0.0066667.
• Number of payments: 5 × 12 = 60.
• Payment: 20,000 × 0.0066667 ÷ [1 − (1 + 0.0066667)⁻⁶⁰] ≈ $405.53 per month.
• Total paid using the displayed payment: $405.53 × 60 = $24,331.80.
• Estimated total interest: $24,331.80 − $20,000 = $4,331.80.

The payment is the scheduled monthly amount for principal and interest under the formula’s assumptions. It is not necessarily the full cost of owning or operating the car.

Assumptions

• The interest rate stays fixed throughout the loan.
• Payments are equal, made monthly, and treated as occurring at the end of each monthly period.
• Interest accrues monthly at APR divided by 12; the APR input is expressed as a percentage per year.
• The estimate excludes fees, taxes, insurance, PMI, escrow, and other charges because they are not calculator inputs.

What to Enter and How to Read the Result

Loan amount means the amount borrowed, not automatically the vehicle’s advertised price. The financed amount may reflect a down payment, trade-in value, or other amounts added to or subtracted from the transaction. For a useful estimate, use the amount that will actually be financed, based on figures provided in the transaction documents.

APR is an annual percentage rate supplied as a percentage, such as 8 for 8%, not 0.08. The calculator converts it to a monthly decimal internally. APR and a nominal interest rate are not always interchangeable in every loan disclosure; use the APR figure specified for the estimate and compare the calculator’s assumptions with the lender’s disclosed terms.

Loan term is entered in years and converted to months. A five-year term means 60 payments. The output titled “Monthly payment” covers only the scheduled loan payment under this calculation. “Total interest” estimates interest across the full schedule; adding the financed principal to that figure gives the estimated total of the rounded monthly payments.

How APR and Loan Term Affect Payments

For a given amount and term, a higher APR produces a higher calculated payment and more interest over the schedule. The term changes the number of payments: extending it generally spreads repayment across more months, which can reduce each scheduled payment while increasing the total interest under the same fixed APR. The exact effect depends on the inputs; the formula does not account for a different rate, fees, or changes to the payment schedule.

To compare hypothetical figures consistently, keep the amount financed and APR the same while changing only the term, or keep the term and amount the same while changing only APR. These are arithmetic comparisons, not predictions of what a lender will offer. Obtain the actual rate, fees, amount financed, and payment schedule from the lender’s disclosure.

Common Mistakes and Limits

• Entering APR in the wrong form: type 8 for 8%, not 0.08. The formula divides the percentage by 1,200 to get a monthly decimal.
• Using the vehicle price as the loan amount: the financed principal can differ from the price. Use the actual amount financed when available.
• Confusing payment with total ownership cost: fuel, maintenance, registration, insurance, and other costs are outside this loan formula.
• Assuming every lender rounds identically: payment rounding, daily interest calculations, payment dates, and fees may produce a different disclosed schedule.

The standard formula is for a fixed rate and equal periodic payments. It does not model variable rates, skipped or irregular payments, extra principal payments, a balloon payment, or fees rolled into the loan unless those amounts are included in the principal. At 0% APR, this particular formula divides by zero; the payment would instead be principal divided by the number of months. The calculator sets a minimum APR of 0.01% to avoid that singular case.

Frequently Asked Questions

How do I calculate a car loan monthly payment with APR?
Convert APR to a monthly decimal by dividing the percentage by 1,200, convert the term to months, then use P × r ÷ [1 − (1 + r)⁻ⁿ]. Here P is the financed amount, r the monthly rate, and n the number of payments.

Does APR get divided by 12 or 1,200?
If APR is entered as a percentage number, such as 8, divide by 1,200 to get a monthly decimal. Dividing 0.08 by 12 gives the same result if the rate was first written as a decimal.

Does the monthly payment include taxes and insurance?
Not in this calculator. It estimates principal and interest only; charges such as taxes, insurance, and fees are excluded unless they are included in the amount financed or added separately by a lender.

Why is my lender’s payment different?
The lender may use a different amount financed, APR, term, payment date, fee treatment, or rounding method. Compare those inputs and the disclosed payment schedule with the calculator’s assumptions.

How does a longer term affect total interest?
At the same principal and fixed APR, a longer term means more monthly periods. It generally lowers the scheduled payment but increases the total interest calculated over the full schedule.

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

Related

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

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