Car Loan Installment Calculator: Monthly Payment & Interest

A car loan installment calculator estimates your fixed monthly payment and total interest from the amount borrowed, annual percentage rate (APR), and loan term. Enter those three figures to see the estimated payment and the interest paid over the full term.

Formula

The calculator uses the standard fixed-payment loan formula. It divides the principal and interest obligation into equal monthly installments, assuming the interest rate remains fixed and payments are made monthly.

• 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 expressed as a percentage, 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. The payment is rounded to cents before total interest is calculated, so the displayed total uses the displayed installment.

Worked Example

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

1. Convert 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 + 0.0066667)⁻⁶⁰) ≈ $405.53 per month, rounded to cents.
4. Calculate total interest using the rounded payment: $405.53 × 60 − $20,000 = $4,331.80.

At these inputs, the estimated monthly installment is $405.53, and estimated total interest is $4,331.80. The sum of the 60 rounded installments is $24,331.80, including the $20,000 principal.

Assumptions

• The interest rate is fixed for the entire term.
• There are equal monthly payments, with the first payment one month after the loan begins.
• Interest compounds monthly at the annual APR divided by 12.
• The estimate excludes separate fees, taxes, insurance, and other charges unless they are included in the amount financed. It does not model add-ons, changing rates, or a different final payment.

What the Estimate Tells You

The monthly payment is the scheduled installment of principal and interest under the formula’s assumptions. Early installments generally include a larger interest portion because interest is calculated on a larger outstanding balance. As the balance declines, more of each level payment goes toward principal. The total-interest figure is the sum of the modeled payments minus the principal; it is not the full cost of owning or operating a car.

Use the amount financed, not automatically the vehicle’s advertised price. A down payment reduces the amount borrowed, while any financed taxes, fees, or optional products increase it. The amount financed should reflect the figures in the lender’s written quote. The calculator cannot determine those figures or tell whether an application will be approved.

How to Use It and Compare Quotes

For an estimate, enter the principal in dollars, APR as a percentage (for example, enter 8 for 8%), and term in years. The term accepts fractions of a year, so six months can be entered as 0.5. Compare estimates only when the principal, APR definition, and term are consistent. If a quote lists a different amount financed or includes charges in the payment, the results will not match this basic calculation.

A longer term usually spreads repayment across more installments, which can lower the scheduled payment while increasing the number of months interest accrues. A shorter term has fewer installments and may have a higher monthly payment. These are mathematical effects, not a judgment about which loan to take. To understand a particular quote, check its payment schedule and disclosure for the actual financed amount, rate, payment count, and any separate charges.

Common Errors and Limits

• Entering APR incorrectly: Enter 8 for 8%, not 0.08. The formula converts the percentage to a monthly decimal rate.
• Using the wrong principal: The car price and the amount financed may differ. Use the loan principal that applies to the estimate.
• Confusing payment with total cost: The monthly result covers modeled principal and interest only. It does not include ongoing vehicle expenses or excluded charges.
• Expecting exact lender rounding: Lenders may calculate daily interest, use different payment dates, or adjust a final installment. Those details can make actual figures differ by a small amount or more.

This calculator assumes a fixed rate and regular monthly installments. It does not model variable rates, skipped or late payments, extra principal payments, fees paid separately, or a payoff before the scheduled end. For a payoff estimate or an amortization schedule, the dates and transaction history may be necessary.

Frequently Asked Questions

How do I calculate a car loan payment?
Use the principal, monthly interest rate (APR divided by 12), and number of monthly payments in the fixed-payment formula. This calculator performs that calculation and rounds the payment to cents.

Does APR mean the monthly interest rate?
No. APR is entered as an annual percentage. In this estimate, the monthly rate is APR divided by 12, with the percentage converted to a decimal by dividing by 100 as well.

Does the monthly installment include insurance and taxes?
Not by default. The result models principal and interest only; taxes, insurance, and separate charges are excluded unless incorporated into the principal you enter.

Why does my lender’s payment differ from this estimate?
The lender may use a different financed amount, payment schedule, interest calculation, rounding method, or included charges. Compare the quote’s disclosed inputs and schedule with the calculator assumptions.

Will a longer car loan always cost less each month?
For the same principal and fixed rate, a longer term spreads repayment across more payments and generally lowers the formula’s monthly installment. It also means interest is calculated over a longer period, so total interest can be higher.

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

Related

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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