Installment Loan Monthly Payment Calculator

This installment loan monthly payment calculator estimates your fixed monthly principal-and-interest 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 interest over the full term.

Formula

The calculator uses the standard fixed-payment amortization formula. It converts APR to a monthly rate, then calculates the payment that repays the principal and interest through equal monthly payments:

Monthly payment = P × r / [1 − (1 + r)⁻ⁿ]

• P is the loan principal, or amount borrowed, in dollars.
• r is the monthly interest rate, calculated as APR ÷ 1,200. For example, an 8% APR gives r = 0.0066667 per month.
• n is the number of monthly payments, calculated as loan term in years × 12.

Estimated total interest is the monthly payment multiplied by the number of payments, minus the original principal: Total interest = monthly payment × n − P. The calculator uses the displayed monthly payment, rounded to cents, for this total. Small differences from a lender’s final figures can result from payment-date conventions, rounding, fees, or other contract details.

Worked Example

Using the calculator’s default inputs—$20,000 borrowed, 8% APR, and a 5-year term:

1. Convert APR to a monthly rate: 8 ÷ 1,200 = 0.0066667.
2. Convert the term to payments: 5 × 12 = 60 monthly payments.
3. Substitute into the formula: $20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰].
4. The monthly payment is approximately $405.53.
5. Using the displayed payment, total of payments is $405.53 × 60 = $24,331.80; subtract $20,000 principal to estimate $4,331.80 in total interest.

Assumptions

• The interest rate is fixed for the full term.
• Payments are equal, made monthly, and begin one month after the loan is issued.
• Interest compounds monthly using APR divided by 12.
• The estimate excludes fees, taxes, insurance, PMI, escrow, late charges, and optional add-ons.

When to Use the Calculator

Use it to understand how the amount financed, APR, and repayment period combine to produce a scheduled payment. It can also help you examine hypothetical scenarios: change one input while keeping the others fixed and observe how the estimate changes. The APR and loan term should come from the lender’s disclosure or offer documents; the default values are only an example, not a statement about available rates or terms.

An installment loan is generally repaid through a set schedule of payments over a defined period. Each payment covers interest accrued for that period and reduces principal. Early in a fully amortizing loan, the interest portion is larger because the outstanding balance is larger; as the balance falls, more of each level payment goes toward principal. The exact allocation depends on the loan’s terms and payment timing.

How to Read the Results

The monthly payment is an estimate of scheduled principal and interest, not necessarily the full amount due each month. A loan may have separately billed charges or fees that this calculator does not model. Total interest is the estimated interest cost across all scheduled payments, assuming every payment is made on time and the loan remains unchanged through the term.

A longer term usually lowers the calculated monthly payment for the same principal and rate, because repayment is spread across more months. It also generally results in more interest overall when the rate and other assumptions remain fixed. A higher APR increases both the payment and total interest. A larger principal increases the payment and, at a fixed APR and term, increases total interest as well.

Common Mistakes and Limits

• Entering the wrong rate format: Enter APR as a percentage, such as 8 for 8%, not 0.08. The calculator converts it to a monthly decimal rate.
• Confusing term units: The term field is in years. A 60-month loan is 5 years, not 60 years.
• Using the purchase price instead of the amount financed: The principal may differ from an advertised price if a down payment, trade-in, financed charges, or other adjustments apply. Use the actual amount borrowed where available.
• Treating payment as total cost: The payment result excludes charges not entered in the calculator. Check which amounts are included in the lender’s stated payment.
• Assuming extra payments are included: Additional principal payments, missed payments, rate changes, and early payoff are not represented by this fixed-schedule calculation.

This formula is for a fully amortizing loan with a positive APR and equal monthly payments. It does not model interest-only periods, balloon payments, variable rates, irregular payment schedules, or fees added to the balance. Since the payment is rounded to cents, a real final payment can differ slightly if the lender adjusts for exact dates or rounding rules.

Frequently Asked Questions

How do I calculate an installment loan payment?
Convert APR to a monthly rate by dividing by 1,200, convert the term to months, then use the fixed-payment formula shown above. The calculator performs these steps and rounds the result to cents.

Does the monthly payment include interest?
Yes. The estimate includes both principal repayment and interest. It does not include fees, insurance, taxes, or other amounts unless they are incorporated into the principal figure you enter.

Why does a longer loan term reduce the monthly payment?
A longer term spreads repayment across more monthly installments. With the same principal and APR, that lowers the scheduled payment, while the additional time generally increases total interest.

Is APR the same as the interest rate?
APR is an annual percentage measure used here as the input for the payment calculation. A lender’s APR may reflect details beyond the stated interest rate, depending on the disclosure. This calculator does not separately model fees or reconstruct a lender’s APR.

Why is my lender’s payment different?
The lender may use a different financed amount, include charges, use different payment dates or rounding, or have terms not represented here. Compare the calculator’s inputs with the loan disclosure and payment schedule.

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

Related

• Google Sheets Accrued Interest Amortization Calculator
• Accrued Interest on Form 1099-INT: What to Report
• Does Accrued Interest Reduce Interest Income?

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