Car Payment Formula: Calculate Your Monthly Payment

The car payment formula calculates a fixed monthly loan payment from the amount financed, annual percentage rate (APR), and loan term. Enter those three figures to estimate the payment and total interest; the estimate excludes costs such as taxes, insurance, and fees unless they are included in the amount financed.

Formula

For a fixed-rate loan repaid in equal monthly installments, the payment is:

Payment = P × r ÷ [1 − (1 + r)⁻ⁿ]

• P is the principal, or amount financed, in dollars.
• r is the monthly interest rate as a decimal: APR ÷ 1200 when APR is entered as a percentage per year.
• n is the number of monthly payments: loan term in years × 12.

The calculator rounds the monthly payment to the nearest cent. It then estimates total interest as the rounded payment multiplied by the number of payments, minus the original principal. The formula assumes a level payment schedule and a positive APR; the calculator accepts APR values from 0.01% to 100% and terms from 0.0833 to 50 years.

Worked Example

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

1. Convert the APR to a monthly decimal rate: 8 ÷ 1200 = 0.0066667.
2. Convert the term to payments: 5 × 12 = 60 months.
3. Substitute in the payment formula: $20,000 × 0.0066667 ÷ [1 − (1 + 0.0066667)⁻⁶⁰].
4. The unrounded result is approximately $405.528; rounded to cents, the monthly payment is $405.53.
5. Estimated total interest: $405.53 × 60 − $20,000 = $4,331.80.

The final payment shown by a lender can differ slightly because payments and interest are rounded according to the contract’s schedule. The example is arithmetic, not a quote.

Assumptions

• The APR stays fixed for the whole term.
• Payments are equal, made monthly, and paid at the end of each monthly period.
• Interest is compounded monthly, using APR divided by 12.
• The calculation does not separately include fees, taxes, insurance, or escrow. Any amount rolled into the loan must be included in the loan amount input.

What Goes Into a Car Payment?

The key input is the amount financed—not necessarily the vehicle’s advertised price. A simplified way to estimate it is to start with the purchase price and eligible items added to the contract, then subtract the down payment, trade-in credit, and any other amount paid upfront. The precise contract amount is the figure to use. A deposit or down payment reduces the amount borrowed; it is not a reduction in the formula’s interest rate.

APR represents the annualized borrowing cost disclosed for the loan, but the calculator uses it as a nominal annual rate divided into monthly periods. Enter the APR quoted for the particular loan, rather than a monthly rate or a rate written as a decimal. For example, enter 8 for 8%, not 0.08 or 0.0067. The term is the number of years, and the formula converts it to monthly payments.

Taxes, registration charges, optional products, and other costs may be paid upfront or included in the financed amount depending on the transaction. If financed, they can increase the payment because the principal is larger. Insurance and ongoing vehicle expenses are not loan payments and are not calculated here.

How to Interpret the Estimate

The monthly payment is the scheduled amount implied by the inputs, not a complete measure of the loan’s cost. The total-interest result estimates how much interest is paid across all scheduled payments under the stated assumptions. Adding the principal to that interest gives the estimated total of payments, but does not account for excluded costs.

For a fixed rate and principal, spreading repayment over more months generally lowers each scheduled payment but allows interest to accrue over a longer period. A shorter term generally means larger monthly payments and fewer periods in which interest accrues. This describes the formula’s mechanics; it does not determine what term or payment is appropriate for any person.

The calculator’s total interest uses the payment rounded to cents, as specified in the output formula. Because a real lender may adjust the final installment to clear a small remaining balance, its exact total can vary by a small amount. The schedule and disclosure provided for the loan are the reference for contractual figures.

Common Mistakes and Limits

• Using the vehicle price instead of the amount financed: subtract amounts paid upfront and account for financed additions to match the contract figure.
• Entering APR in the wrong form: type a percentage such as 8, not 0.08. The formula divides by 1200 to get a monthly decimal rate.
• Mixing up months and years: this calculator asks for years and multiplies by 12. If a quoted term is in months, convert it to years before entering it.
• Treating the estimate as a complete ownership budget: fuel, maintenance, insurance, and other running costs are outside the payment formula.

The standard formula is not designed for variable rates, irregular payment dates, balloon payments, payment deferrals, or fees that change the repayment schedule. A zero-interest loan also requires a separate zero-rate calculation; this calculator’s APR minimum is 0.01%. If the actual contract has different timing or terms, its payment schedule cannot be reproduced fully by this simple estimate.

Frequently Asked Questions

What is the formula for a monthly car payment?
Multiply the amount financed by the monthly rate, then divide by 1 minus (1 plus the monthly rate) raised to the negative number of monthly payments. The monthly rate is APR ÷ 1200.

How do I calculate a car payment by hand?
Convert APR to a monthly decimal, multiply the loan term in years by 12, and substitute both values and the principal into the formula. Round the result to the nearest cent.

Does a down payment affect the monthly payment?
It can reduce the amount financed. Use the loan principal after the down payment and other upfront amounts have been accounted for, rather than the vehicle price alone.

Does this car payment formula include insurance and taxes?
No. It calculates principal and interest for the amount entered. Include a cost in the principal only if it is financed; separately paid insurance, taxes, and fees are not automatically added.

Why might my lender’s payment differ from this estimate?
The loan may use a different financed amount, APR, term, payment timing, or fee structure. Rounding and a slightly adjusted final installment can also change the exact figures.

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

Related

• Personal Loan Amortization Schedule Calculator
• Loan Repayment Amortization Calculator: Payments and Interest
• Loan Amortization XLS: Calculate Payments and Balances

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