Auto Interest Loan Calculator and Payment Formula

This auto loan calculator estimates the monthly payment and total interest for a fixed-rate car loan. Enter the amount financed, annual percentage rate (APR), and term in years to see the scheduled payment and the interest implied by those inputs.

Formula

The calculator uses the standard fixed-payment amortization formula. It first calculates a monthly payment, rounded to the nearest cent, then subtracts the amount borrowed from all scheduled payments to estimate total interest.

• Monthly payment: M = round(P × r ÷ [1 − (1 + r)⁻ⁿ], 2)
• Total interest: I = M × n − P
• Monthly rate: r = APR ÷ 1,200
• Number of payments: n = loan term in years × 12

P is the amount financed in dollars, APR is the annual percentage rate entered as a percent, r is the monthly rate as a decimal, n is the number of monthly payments, M is the monthly payment in dollars, and I is total interest in dollars. For example, an 8% APR becomes 0.08 ÷ 12 per month. The formula assumes equal monthly payments and a constant rate.

Worked Example

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

1. Convert the APR to a monthly decimal rate: 8 ÷ 1,200 = 0.0066667.
2. Convert the term to payments: 5 × 12 = 60 months.
3. Apply the payment formula: $20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰] = $405.53 per month, rounded to cents.
4. Calculate interest using the rounded payment: $405.53 × 60 − $20,000 = $4,331.80.

The calculator therefore displays a monthly payment of $405.53 and total interest of $4,331.80 for these inputs. The example is arithmetic, not a quote or a statement about rates or loan terms available to a borrower.

Assumptions

• The APR and scheduled payment remain fixed for the full term.
• Payments are made monthly, with the first payment one month after the loan begins.
• Interest accrues monthly at APR divided by 12.
• The amount entered is the full principal financed; the calculation does not separately model fees, taxes, insurance, add-ons, a down payment, or a trade-in.
• There are no extra payments, late payments, skipped payments, or changes to the schedule.

What Auto Loan Interest Means

Auto loan interest is the cost of borrowing the principal used to purchase a vehicle. A lender’s payment schedule generally applies each payment first to interest accrued for the period and then to the remaining principal. Early in a typical fully amortizing loan, the balance is larger, so a greater share of a payment goes toward interest. As the balance falls, the interest portion usually falls and more of each level payment reduces principal.

APR is an annualized measure supplied in the loan terms. It is not the monthly rate: this calculator converts it to a monthly rate by dividing by 12. A lender’s APR calculation or payment schedule may account for details this simplified estimate does not model, so use the figures from the actual disclosure for a specific offer.

What Amount Should You Enter?

Enter the amount financed, not automatically the vehicle’s advertised or negotiated price. The amount financed can be affected by a down payment, trade-in credit, taxes, fees, or optional products included in the contract. The calculator has no separate fields for these items, so reflect them in the principal only if you know the final amount being financed.

For instance, if a vehicle purchase has a price of $24,000 and the final contract finances $19,000 after money paid up front and other adjustments, enter $19,000. Use the APR and term shown in the loan offer or disclosure rather than relying on a general estimate. If fees are paid separately rather than financed, they are not included in this calculator’s interest estimate.

How to Interpret and Compare Estimates

The monthly payment is the scheduled amount under the stated assumptions. Total interest is the sum of those rounded monthly payments minus the principal. It is not the total amount paid: total scheduled payments are the payment multiplied by the number of months, while the principal is the amount originally borrowed.

When exploring scenarios, change one input at a time to see how the arithmetic responds. A larger principal or longer term generally increases the total amount of interest when the APR is above zero. Extending the term can lower the payment while increasing the number of periods in which interest accrues. A different APR also changes both the payment and interest. These are mathematical relationships, not guidance about which loan to select.

Comparisons are meaningful only when the inputs describe comparable amounts, rates, and repayment periods. A monthly payment alone does not show the full cost: a lower payment may result from a longer term, and the calculator does not include charges outside its inputs. Check the lender’s itemized figures and payment schedule to understand the complete transaction.

Limits and Common Mistakes

• Confusing APR and monthly rate: Enter APR as a percentage such as 8, not 0.08 and not the monthly rate.
• Using vehicle price instead of principal: The formula needs the amount financed after applicable adjustments.
• Assuming every contract behaves identically: Variable rates, irregular payment dates, fees, add-ons, or payment changes are outside this fixed-rate model.
• Overlooking cent rounding: The payment is rounded to cents before total interest is calculated. A lender may handle final-payment rounding or dates differently.
• Treating estimates as approval or an offer: The calculator does not assess eligibility, underwriting, a specific contract, or a lender’s exact calculation.

Zero-interest loans are not accepted by this calculator because its payment formula divides by the monthly rate; do not enter zero as an APR. For a genuinely interest-free loan, the simple payment would be principal divided by the number of payments, but other contract terms may still affect payment timing or amounts.

Frequently Asked Questions

How do I calculate interest on a car loan?
For a fixed-rate loan with equal monthly payments, calculate the payment using the amortization formula, then multiply that rounded payment by the number of months and subtract the principal. This calculator performs those steps.

Does a longer auto loan term mean more interest?
With the same principal and a positive fixed APR, a longer term usually means more scheduled payments and more total interest, even though the monthly payment is generally lower. The precise estimate depends on the inputs.

Is APR the same as the monthly interest rate?
No. APR is entered as an annual percentage. This calculator divides it by 1,200 to obtain the monthly rate as a decimal for the formula.

Does the calculator include taxes and fees?
No. It calculates payment and interest from the principal, APR, and term only. If an amount is included in the financed principal, its effect is reflected indirectly, but the calculator does not identify or itemize it.

Why might a lender’s payment differ?
A contract may use a different financed amount, payment dates, fee treatment, rounding method, or other terms. The lender’s disclosure and payment schedule give the figures for that specific contract.

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

Related

• Advanced Loan Payment Calculator: Payments and Interest
• Calculate a Monthly Loan Payment: Formula and Example
• How to Calculate Loan Payments: Formula and Example

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 | $ | round(p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2