Auto Loan and Amortization Calculator

This auto loan and amortization calculator estimates a fixed-rate car loan’s monthly payment and total interest from the amount financed, APR, and term. Enter those figures to see the payment and the interest implied by a schedule of equal monthly payments.

Formula

The calculator uses the standard fixed-payment loan formula. It converts annual APR to a monthly rate, then calculates the payment that repays principal and interest over the selected number of months:

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

P is the amount financed in dollars; APR is the annual percentage rate entered as a percent; r is the monthly rate, APR ÷ 1200; n is the number of monthly payments, years × 12; M is the monthly payment in dollars; and I is total interest in dollars. The calculator displays the payment to cents. Its total-interest expression uses that displayed, cent-rounded payment.

Worked Example

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

1. Convert APR to a monthly rate: r = 8 ÷ 1200 = 0.0066667.
2. Convert the term to payments: n = 5 × 12 = 60.
3. Substitute into the payment formula: M = 20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰] ≈ $405.53 per month.
4. Calculate total interest using the displayed payment: I = $405.53 × 60 − $20,000 = $4,331.80.

The results are an estimated monthly payment of $405.53 and estimated total interest of $4,331.80. Actual payment schedules may differ because lenders can calculate, collect, or round amounts differently.

Assumptions

• The interest rate is fixed for the entire term.
• Payments are equal, made monthly, with the first payment one month after the loan begins.
• Interest accrues monthly at APR ÷ 12. The model treats APR as the nominal annual rate for this calculation.
• The estimate excludes fees, taxes, insurance, optional products, and other charges unless they are included in the amount financed. It does not model escrow, down-payment timing, or a changing balance caused by extra payments.

What an Auto Loan Amortization Schedule Shows

An amortizing loan is repaid through scheduled payments that cover interest and reduce principal. Early in the schedule, the outstanding balance is larger, so more of each payment goes toward interest. As principal declines, the interest portion generally shrinks and more of the same payment reduces principal. The final scheduled payment brings the modeled balance to zero, subject to rounding.

The calculator provides the payment and lifetime interest, not a month-by-month table. To understand the pattern, the balance after k payments can be calculated as P(1 + r)^k − M[(1 + r)^k − 1] ÷ r, where k is the number of payments already made. For example, the balance after the first payment is not simply the original amount minus the full payment: that month’s interest is due first.

What Amount Should You Enter?

Enter the amount actually financed, rather than assuming it is the vehicle’s advertised price. A vehicle transaction can involve several components, and the lender’s documents determine which are included in the loan balance. A down payment or trade-in may reduce the amount financed; taxes, fees, or optional items may increase it if they are rolled into the loan. Use the figure supplied for the loan amount by the lender or in the proposed loan disclosure.

Enter the APR and term as annual percent and years, respectively. For instance, enter 8 for an 8% APR, not 0.08; the formula divides the percentage by 1200 to obtain a monthly decimal rate. The term is converted to months, so five years means 60 payments. The APR and loan terms in the example are calculation inputs, not claims about available offers. Get the actual rate, amount, and payment schedule from the lender.

How to Read and Use the Estimate

The monthly payment isolates the scheduled loan payment under the stated assumptions. It is not necessarily the full cost of having or operating a vehicle: insurance, fuel, maintenance, registration, and other ownership costs are outside this formula. Likewise, total interest is the modeled interest across the full term, not the total amount paid; adding principal gives the total of modeled payments.

Changing one input can help show how loan structure affects the arithmetic. A larger amount financed increases the payment and interest when other inputs stay fixed. A longer term generally lowers the required monthly payment but allows more months for interest to accrue. A higher APR increases the payment and interest for a given amount and term. These are mathematical relationships, not guidance about which terms to select or whether a particular payment fits someone’s finances.

Common Calculation Limits and Mistakes

• Confusing price and principal: the amount financed can differ from the vehicle’s price because of a down payment, trade-in, or financed charges.
• Entering APR as a decimal: use 8 for 8%, not 0.08. The calculator handles the percentage conversion.
• Assuming all charges are included: this estimate does not add costs that are not represented in the entered principal or formula.
• Expecting exact lender figures: cent rounding, payment dates, daily interest methods, and contract terms can change actual amounts. The formula is a standard monthly estimate.
• Assuming extra payments are modeled: the calculation assumes scheduled equal payments only. It does not account for extra principal payments, skipped or late payments, fees, or refinancing.

The formula is designed for a positive APR and a fixed-rate, fully amortizing loan with regular monthly payments. It does not determine approval, include lender-specific terms, or calculate a payoff quote for a particular date.

Frequently Asked Questions

How do I calculate an auto loan payment?
Convert APR to a monthly decimal rate by dividing by 1200, convert the term to months, and apply M = P × r ÷ [1 − (1 + r)⁻ⁿ]. The calculator performs those steps and rounds the payment to cents.

Does total interest include the amount borrowed?
No. Total interest is the modeled cost of borrowing. Add the amount financed to total interest to get the sum of the calculator’s scheduled payments.

Why does a longer auto loan have a lower monthly payment?
It spreads repayment across more monthly payments. Since the balance remains outstanding for longer, total interest can be higher even when the monthly payment is lower.

Does this calculator include a down payment or trade-in?
Only indirectly. Enter the resulting amount financed after any down payment or trade-in is accounted for. The calculator does not separately calculate vehicle price, trade-in value, or cash due at signing.

Why is my lender’s payment different?
The contract may use different rounding, payment dates, financed charges, or interest-accrual details. Compare the loan amount, APR, term, and included costs, then refer to the lender’s payment schedule for contract figures.

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