Car Payment Amortization Schedule: Formula and Calculator

A car payment amortization schedule shows how each monthly payment is divided between interest and principal, and how the loan balance changes over time. Enter the amount financed, APR, and term below to estimate the monthly payment and total interest; the schedule itself can be built by applying the same balance calculation to each month.

Formula

The calculator uses the standard fixed-payment loan formula. It assumes equal monthly payments and a constant APR, with interest accruing monthly.

• Monthly payment: M = P × r ÷ [1 − (1 + r)⁻ⁿ]
• Total interest: I = M × n − P

Here, P is the amount financed in dollars, APR is the annual percentage rate entered as a percentage, r = APR ÷ 1,200 is the monthly rate as a decimal, n is the term in months (years × 12), M is the monthly payment in dollars, and I is estimated total interest in dollars. The displayed monthly payment is rounded to cents; the total-interest output uses that displayed payment multiplied by the number of months, then subtracts the amount financed.

Worked Example

Using the calculator’s default inputs—$20,000 financed, 8% APR, and five years:

1. Convert the annual rate to a monthly decimal: r = 8 ÷ 1,200 = 0.0066667.
2. Convert the term to months: n = 5 × 12 = 60.
3. Apply the payment formula: M = 20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰].
4. The monthly payment is approximately $405.53 after rounding to cents.
5. Total interest is $405.53 × 60 − $20,000 = $4,331.80.

Assumptions

• The APR remains fixed for the entire term.
• Payments are equal, made monthly, and treated as occurring at the end of each monthly period.
• Interest compounds monthly at APR ÷ 12.
• The calculation excludes fees, taxes, insurance, and other charges unless they are included in the amount financed. It does not model a down payment separately.

How a Car Amortization Schedule Works

An amortization schedule is a month-by-month ledger. At the start of each period, interest is calculated on the outstanding balance. The payment first covers that period’s interest; whatever remains reduces principal. The next period’s interest is then calculated on the smaller balance.

For a schedule with monthly rate r, opening balance B, and payment M:

• Interest for the month = B × r.
• Principal repaid = M − interest for the month.
• Closing balance = B − principal repaid.

For the example, first-month interest is about $20,000 × 0.0066667 = $133.33. From the $405.53 payment, about $272.20 reduces principal, leaving an estimated balance of $19,727.80. In later months, the interest portion generally declines while the principal portion rises. Small differences can occur because real lenders may calculate interest using specific dates, daily accrual, and their own rounding procedures.

Building and Reading the Schedule

To create a complete estimate, repeat the three balance steps for each payment period. A useful table has columns for payment number, opening balance, payment, interest, principal, and closing balance. The opening balance for the next row is the prior row’s closing balance.

Check the schedule’s final row: after the last scheduled payment, the remaining balance should be close to zero. It may not be exactly zero in a hand-built table because each row is rounded. For an actual loan, the final payment can differ slightly to clear the remaining amount. Do not use a rounded estimate as a payoff quote; request the exact payoff figure from the lender.

What Changes the Payment and Total Interest

The amount financed, APR, and term all affect the result. A larger amount financed raises the payment when other inputs stay the same. A higher APR raises the interest charged and generally raises both the payment and total interest. A longer term usually lowers the required monthly payment but means more payment periods, so total interest can be higher when the APR and amount are unchanged.

The amount financed may not equal the vehicle’s advertised price. A down payment or trade-in can reduce it, while financed taxes, fees, optional products, or a prior balance rolled into the new loan can increase it. Enter the amount the loan actually finances, not a price figure that omits amounts included in the contract. Obtain the APR, term, and financed amount from the lender’s written figures rather than assuming an advertised or illustrative number applies.

Common Misreadings and Limits

A payment estimate is not the same as the full cost of owning a vehicle. The calculator does not include insurance, fuel, maintenance, registration, or charges that are paid outside the loan. It also cannot determine approval, lender-specific fees, or the precise amount required to settle a loan on a particular date.

Another common misconception is that the interest portion is a fixed fraction of every payment. It is not: under this fixed-rate model, monthly interest is based on the changing balance. Also, APR is entered as a percent, so 8 means 8%, not 0.08; the formula converts it by dividing by 1,200. Rounding every schedule row can create small discrepancies, so retain extra decimal places during intermediate calculations and round only for display when possible.

Frequently Asked Questions

What is a car payment amortization schedule?
A table showing each payment’s interest and principal portions, along with the remaining loan balance after each payment.

How do I calculate the interest portion of a car payment?
Multiply the period’s opening balance by the monthly interest rate. Subtract that interest from the payment to estimate the principal repaid.

Why does more of my payment go to principal later?
As principal is paid down, the balance on which interest is calculated falls. With a fixed payment, that usually leaves more of each later payment available for principal.

Does a longer car loan term reduce total interest?
It can reduce the scheduled monthly payment, but for the same amount financed and APR, a longer term generally adds payment periods and increases total interest.

Why does my lender’s schedule differ from this estimate?
The lender may use exact payment dates, daily interest accrual, contract-specific rounding, fees, or a final adjusted payment. The calculator uses a simplified monthly model.

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