Simple Interest Auto Loan Calculator Payment

This simple interest auto loan calculator estimates the fixed monthly payment and total interest for a car loan when you enter the amount financed, annual percentage rate (APR), and term in years. It uses a standard fixed-payment formula; the actual amount and timing of interest on an auto loan can differ under the lender’s contract.

Formula

The calculator finds a level monthly payment using the standard amortizing-loan formula:

Payment = P × r ÷ [1 − (1 + r)⁻ⁿ]

• P is the amount borrowed, in dollars.
• r is the monthly interest rate as a decimal: annual APR ÷ 100 ÷ 12. With APR entered as a percentage, this is APR ÷ 1200.
• n is the number of monthly payments: loan term in years × 12.

Total interest is estimated as the rounded monthly payment multiplied by the number of payments, minus the original loan amount: interest = payment × n − P. The displayed payment is rounded to cents before calculating total interest.

Worked Example

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

1. Convert the APR to a monthly decimal rate: 8 ÷ 1200 = 0.0066667.
2. Find the number of payments: 5 × 12 = 60.
3. Substitute into the payment formula: $20,000 × 0.0066667 ÷ [1 − (1 + 0.0066667)⁻⁶⁰].
4. The monthly payment, rounded to cents, is $405.53.
5. Calculate total interest using that rounded payment: $405.53 × 60 − $20,000 = $4,331.80.

Assumptions

• The APR is fixed for the full term.
• Payments are equal, made monthly, and begin on a regular schedule.
• Interest is modelled monthly at APR ÷ 12.
• Fees, taxes, insurance, optional products, and payment-date variations are not included.

How Simple Interest Works on an Auto Loan

With a simple-interest auto loan, interest is generally calculated from the outstanding principal balance rather than added in advance as a fixed charge. As payments reduce principal, the balance on which later interest is calculated falls. The amount of interest accruing between payments can therefore depend on the balance and the number of days between payment dates.

The calculator uses a conventional monthly amortization approximation. It treats each period as one month and applies a constant monthly rate. A lender may instead calculate daily interest using the contract’s day-count method, so the precise interest charged can vary with the dates payments are received. The calculator is useful for an estimate, not a reconstruction of a lender’s account ledger.

How to Use and Read the Estimate

Enter the amount financed—not necessarily the vehicle’s advertised price—then enter the APR and the term in years. The amount financed may reflect a down payment, trade-in value, or other amounts included in the contract. Use figures from a written lender quote or disclosure when available; do not assume that a quoted rate or price applies to your situation.

• Monthly payment is the calculated principal-and-interest payment under the formula’s assumptions.
• Total interest is the estimated interest across all scheduled payments. It is not the total amount paid; add the principal to get the estimated total of payments.
• Term affects both payment size and the number of payments. Changing it changes the calculation, but the calculator does not assess whether a particular term fits a person’s circumstances.

For example, a longer term spreads repayment over more months. That can lower the formula’s monthly payment for the same principal and APR, while increasing the number of periods over which interest is charged. Actual comparisons should use consistent loan amounts and lender-provided contract figures.

Common Mistakes and Limits

• Mixing APR and monthly rates: Enter annual APR as a percentage, such as 8—not 0.08 and not a monthly rate. The calculator converts it to a monthly decimal internally.
• Using the vehicle price instead of the loan amount: Enter the principal actually financed. The purchase price and amount borrowed may not be the same.
• Expecting daily-interest precision: The formula assumes equal monthly periods. Payment timing and daily accrual can make a contract’s interest differ.
• Forgetting excluded charges: The estimate does not add fees, taxes, insurance, or optional products unless included in the amount financed you enter. It also does not model late payments, extra principal payments, or a changing rate.
• Rounding differences: The payment is rounded to the nearest cent, and total interest uses that rounded payment. A lender’s final payment may differ because of its exact dates and rounding practices.

A zero-interest loan is not supported by this formula as written because its payment expression divides by the monthly rate. The APR input therefore has a minimum of 0.01%. If the contract has no interest, dividing principal by the number of payments gives a basic principal-only payment, but other contract details may still affect amounts due.

Frequently Asked Questions

How do I calculate a simple interest car loan payment?
Convert annual APR to a monthly decimal by dividing by 1200, multiply the loan amount by that rate, and divide by 1 minus (1 plus the rate) raised to the negative number of monthly payments. This calculator performs those steps and rounds the payment to cents.

Is a simple-interest auto loan payment the same every month?
A scheduled payment can be level, while the interest portion and principal portion change over time. With daily simple-interest accrual, the interest amount for a period may also vary with payment timing, depending on the contract.

Does a longer auto loan term reduce total interest?
Not necessarily. For the same amount and APR, a longer term generally means more payment periods, which can increase total interest under the fixed-rate formula even though the calculated monthly payment is lower.

Why is my lender’s payment different from this estimate?
The contract may use daily interest, different payment dates, a different financed amount, fees, optional items, or another rounding method. Compare the calculator inputs with the lender’s disclosed loan amount, APR, term, and payment schedule.

Does the estimate include taxes, insurance, or fees?
No. They are not separate calculator inputs. If a charge is included in the amount financed, it affects the calculated payment indirectly; otherwise, it is outside this estimate.

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

Related

• Loan Calculator With Payment Amount and Total Interest
• Monthly Loan Repayment Calculator and Payment Formula
• 30 Year Loan Repayment Calculator: Monthly Payment & Interest

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